fix: make migrations applicable on existing data.

During adaptation for PostgreSQL, the modifications were made on the existing changesets. It has
made them failing on existing data as the checksums were changed. In the new approach, we moved
conditions from statements within a changeset to a changeset itself.

Correction for the following commits:
- c03b2cc
- 6d296fd
- 7282a7b
- d767dab
- 11c898b

Addressed to #1034

Author: php-coder

src/main/resources/liquibase/version/0.3/2014-05-28--release_month_and_day.xml

@@ -32,16 +32,21 @@
 
 	</changeSet>
 
-	<changeSet id="fill-release_year-column-based-on-values-from-released_at" author="php-coder" context="test-data, prod-data">
+	<changeSet id="fill-release_year-column-based-on-values-from-released_at" author="php-coder" context="test-data, prod-data" dbms="mysql,h2">
 		<comment>Migrates data from series.released_at to series.release_year</comment>
 
-		<sql dbms="mysql,h2">
+		<sql>
 			UPDATE series
 			SET release_year=YEAR(released_at)
 			WHERE released_at IS NOT NULL
 		</sql>
 
-		<sql dbms="postgresql">
+	</changeSet>
+	
+	<changeSet id="fill-release_year-column-based-on-values-from-released_at" author="php-coder" context="test-data, prod-data" dbms="postgresql">
+		<comment>Migrates data from series.released_at to series.release_year</comment>
+		
+		<sql>
 			UPDATE series
 			SET release_year=EXTRACT(YEAR FROM released_at)
 			WHERE released_at IS NOT NULL

src/main/resources/liquibase/version/0.3/2014-09-28--collection_slug.xml

@@ -14,11 +14,11 @@
 
 	</changeSet>
 
-	<changeSet id="update-collections-slug" author="php-coder" context="test-data,prod-data">
+	<changeSet id="update-collections-slug" author="php-coder" context="test-data,prod-data" dbms="mysql,h2">
 		<comment>Sets value of slug field to transformed collection's owner's name</comment>
 
 		<!-- Unfortunately, H2 doesn't support UPDATE with JOIN -->
-		<sql dbms="mysql,h2">
+		<sql>
 			UPDATE collections c
 			SET c.slug = (
 				SELECT LOWER(REPLACE(u.login, ' ', '-'))
@@ -27,8 +27,13 @@
 			);
 		</sql>
 
+	</changeSet>
+	
+	<changeSet id="update-collections-slug" author="php-coder" context="test-data,prod-data" dbms="postgresql">
+		<comment>Sets value of slug field to transformed collection's owner's name</comment>
+		
 		<!-- In PostgreSQL it is illegal to prefix columns with table alias in the SET clause -->
-		<sql dbms="postgresql">
+		<sql>
 			UPDATE collections c
 			SET slug = (
 				SELECT LOWER(REPLACE(u.login, ' ', '-'))

src/main/resources/liquibase/version/0.4/2015-06-22--image_url.xml

@@ -23,7 +23,7 @@
 
 	</changeSet>
 
-	<changeSet id="fill-series_images-table-based-on-values-from-image_url" author="php-coder" context="test-data, prod-data">
+	<changeSet id="fill-series_images-table-based-on-values-from-image_url" author="php-coder" context="test-data, prod-data" dbms="mysql,h2">
 		<preConditions>
 			<sqlCheck expectedResult="0">
 				SELECT COUNT(*)
@@ -34,14 +34,27 @@
 
 		<comment>Migrates data from series.image_url to series_images</comment>
 
-		<sql dbms="mysql,h2">
+		<sql>
 			INSERT INTO series_images(series_id, image_id)
 			SELECT id AS series_id, REPLACE(image_url, '/image/', '') AS image_id
 			FROM series
 		</sql>
 
+	</changeSet>
+	
+	<changeSet id="fill-series_images-table-based-on-values-from-image_url" author="php-coder" context="test-data, prod-data" dbms="postgresql">
+		<preConditions>
+			<sqlCheck expectedResult="0">
+				SELECT COUNT(*)
+				FROM series
+				WHERE image_url IS NULL
+			</sqlCheck>
+		</preConditions>
+		
+		<comment>Migrates data from series.image_url to series_images</comment>
+		
 		<!-- PostgreSQL requires us to cast a string to an integer explicitly -->
-		<sql dbms="postgresql">
+		<sql>
 			INSERT INTO series_images(series_id, image_id)
 			SELECT id AS series_id, CAST(REPLACE(image_url, '/image/', '') AS INT) AS image_id
 			FROM series

src/main/resources/liquibase/version/0.4/2015-07-07--salt_and_hash.xml

@@ -12,23 +12,24 @@
 
 	</changeSet>
 
-	<changeSet id="increase-length-of-hash-column-in-users-table" author="php-coder" context="scheme">
+	<changeSet id="increase-length-of-hash-column-in-users-table" author="php-coder" context="scheme" dbms="mysql">
 		<comment>Increases length of hash column in users table</comment>
 
 		<!--
 			We can't use modifyDataType because it looses not null setting:
 			http://forum.liquibase.org/topic/warning-of-losing-primary-key-autoincrement-for-mysql
 		-->
-		<sql dbms="mysql,h2">
+		<sql>
 			ALTER TABLE users
 			MODIFY COLUMN hash VARCHAR(60) NOT NULL
 		</sql>
 
-		<sql dbms="postgresql">
-			ALTER TABLE users
-			ALTER COLUMN hash
-			TYPE VARCHAR(60)
-		</sql>
+	</changeSet>
+	
+	<changeSet id="increase-length-of-hash-column-in-users-table" author="php-coder" context="scheme" dbms="postgresql,h2">
+		<comment>Increases length of hash column in users table</comment>
+		
+		<modifyDataType tableName="users" columnName="hash" newDataType="VARCHAR(60)" />
 
 	</changeSet>
 

src/main/resources/liquibase/version/0.4/2018-06-09--add_number_of_stamps_to_collections_series.xml

@@ -13,10 +13,10 @@
 
 	</changeSet>
 
-	<changeSet id="fill-collections_series-number_of_stamps-field" author="php-coder" context="prod-data">
+	<changeSet id="fill-collections_series-number_of_stamps-field" author="php-coder" context="prod-data" dbms="mysql,h2">
 		<comment>Sets value of number_of_stamps field to number of stamps in a series</comment>
 
-		<sql dbms="mysql,h2">
+		<sql>
 			UPDATE collections_series cs
 			SET cs.number_of_stamps = (
 				SELECT s.quantity
@@ -25,8 +25,20 @@
 			)
 		</sql>
 
+		<rollback>
+			<sql>
+				UPDATE collections_series
+				SET number_of_stamps = NULL
+			</sql>
+		</rollback>
+		
+	</changeSet>
+	
+	<changeSet id="fill-collections_series-number_of_stamps-field" author="php-coder" context="prod-data" dbms="postgresql">
+		<comment>Sets value of number_of_stamps field to number of stamps in a series</comment>
+		
 		<!-- In PostgreSQL it is illegal to prefix columns with table alias in the SET clause -->
-		<sql dbms="postgresql">
+		<sql>
 			UPDATE collections_series cs
 			SET number_of_stamps = (
 				SELECT s.quantity