* Initial work on #15621
* Signal receiver should ignore models which don't support notifications
* Flesh out NotificationGroup functionality
* Add NotificationGroup filters for users & groups
* Separate read & dimiss actions
* Enable one-click dismissals from notifications list
* Include total notification count in dropdown
* Drop 'kind' field from Notification model
* Register event types in the registry; add colors & icons
* Enable event rules to target notification groups
* Define dynamic choices for Notification.event_name
* Move event registration to core
* Add more job events
* Misc cleanup
* Misc cleanup
* Correct absolute URLs for notifications & subscriptions
* Optimize subscriber notifications
* Use core event types when queuing events
* Standardize queued event attribute to event_type; change content_type to object_type
* Rename Notification.event_name to event_type
* Restore NotificationGroupBulkEditView
* Add API tests
* Add view & filterset tests
* Add model documentation
* Fix tests
* Update notification bell when notifications have been cleared
* Ensure subscribe button appears only on relevant models
* Notifications/subscriptions cannot be ordered by object
* Misc cleanup
* Add event icon & type to notifications table
* Adjust icon sizing
* Mute color of read notifications
* Misc cleanup
* Introduce custom form widget templates to apply CSS classes
* Apply both mandatory and optional CSS classes to form widgets
* Omit required & placeholder attrs
* Move annotation of field validation failures to CSS
* Remove BootstrapMixin class
* Remove obsolete ComponentTemplateImportForm class
* Remove obsolete custom forms for login & password change
* Clean up obsolete accommodations for 'required' widget attr
