Command contact_owners: add support to filter by usernames

Add `--usernames` argument to be able to filter by usernames when sending
contacting users.

This can be used as:

```
django-admin contact_owners --notification notification.md --sticky --usernames usernames.txt
```

```
humitos
santos
anthony
```

```
Read the Docs is going to force email verification to be able to login accounts in the following weeks.
We've noticed **your account doesn't have any email verified**.
Please, go to [your email settings](/accounts/email/) and verify your email now.
```

In this case, it will send a sticky notification with that content to these 3 users.

Required by #9865

Author: humitos

readthedocs/core/management/commands/contact_owners.py

@@ -1,8 +1,8 @@
-import structlog
 import sys
 from pathlib import Path
 from pprint import pprint
 
+import structlog
 from django.conf import settings
 from django.contrib.auth import get_user_model
 from django.core.management.base import BaseCommand
@@ -31,10 +31,16 @@ class Command(BaseCommand):
 
     Email and send an ephemeral (disappears after shown once) notification to all owners of the "readthedocs" organization::
 
-      django-admin contact_owners --email email.md --notification notification.md --organization readthedocs  # noqa
+      django-admin contact_owners --email email.md --notification notification.md --organization readthedocs
+
+    Send a sticky notifications to multiple users::
 
-    Where ``email.md`` is a markdown file with the first line as the subject, and the rest is the content.
-    ``user`` and ``domain`` are available in the context.
+      django-admin contact_owners --notification notification.md --sticky --usernames usernames.txt
+
+    * ``usernames.txt`` is a text file containing one username per line.
+    * ``notifications.md`` is a Markdown file containing the message to be included in the notification.
+    * ``email.md`` is a Markdown file with the first line as the subject, and the rest is the content.
+      Note that ``user`` and ``domain`` are available in the context.
 
     .. code:: markdown
 
@@ -50,7 +56,7 @@ class Command(BaseCommand):
        add the ``--production`` flag to actually send the email/notification.
     """
 
-    help = 'Send an email or sticky notification from a file (markdown) to all owners.'
+    help = "Send an email or sticky notification from a file (Markdown) to users."
 
     def add_arguments(self, parser):
         parser.add_argument(
@@ -92,16 +98,24 @@ def add_arguments(self, parser):
             '--project',
             help='Project slug to filter by.',
         )
+        parser.add_argument(
+            "--usernames",
+            help="Path to a file with the one username per line to filter by.",
+        )
 
     def handle(self, *args, **options):
         if not options['email'] and not options['notification']:
             print("--email or --notification is required.")
             sys.exit(1)
 
-        project = options['project']
-        organization = options['organization']
-        if project and organization:
-            print("--project and --organization can\'t be used together.")
+        project = options["project"]
+        organization = options["organization"]
+        usernames = options["usernames"]
+        if (
+            len([item for item in [project, organization, usernames] if bool(item)])
+            >= 2
+        ):
+            print("--project, --organization and --usernames can't be used together.")
             sys.exit(1)
 
         if project:
@@ -116,6 +130,15 @@ def handle(self, *args, **options):
                 .filter(organizationowner__organization__disabled=False)
                 .distinct()
             )
+        elif usernames:
+            file = Path(usernames)
+            with file.open() as f:
+                usernames = f.readlines()
+
+            # remove "\n" from lines
+            usernames = [line.strip() for line in usernames]
+
+            users = User.objects.filter(username__in=usernames)
         else:
             users = (
                 User.objects
@@ -124,16 +147,17 @@ def handle(self, *args, **options):
             )
 
         print(
-            'len(owners)={} production={} email={} notification={}'.format(
+            "len(owners)={} production={} email={} notification={} sticky={}".format(
                 users.count(),
-                bool(options['production']),
-                options['email'],
-                options['notification'],
+                bool(options["production"]),
+                options["email"],
+                options["notification"],
+                options["sticky"],
             )
         )
 
-        if input('Continue? y/n: ') != 'y':
-            print('Aborting run.')
+        if input("Continue? y/N: ") != "y":
+            print("Aborting run.")
             return
 
         notification_content = ''