Addons: split single checkbox into two

Instead of showing one checkbox to enable "Show notification on latest and
non-stable versions" we have two checkboxes now:

- "Show notification on latest version"
- "Show notification on non-stable versions"

Requires readthedocs/addons#414
Requires readthedocs/readthedocs.org#11718

Author: humitos

readthedocsext/theme/templates/projects/addons_form.html

@@ -76,8 +76,10 @@
 
       {% block addons_notifications %}
         <div class="ui tab" data-tab="notifications">
-          {{ form.external_version_warning_enabled | as_crispy_field }}
-          {{ form.stable_latest_version_warning_enabled | as_crispy_field }}
+          {{ form.notifications_enabled | as_crispy_field }}
+          {{ form.notifications_show_on_external | as_crispy_field }}
+          {{ form.notifications_show_on_latest | as_crispy_field }}
+          {{ form.notifications_show_on_non_stable | as_crispy_field }}
         </div>
       {% endblock addons_notifications %}