Fallback to the associated user when username is missing from job
lifecycle event contexts. Add a regression test to ensure JOB_COMPLETED
webhooks are enqueued without a request context.
Fixes#21371
* Closes#20003: Introduce mechanism to register callbacks for webhook context
* Swap ContentType with ObjectType
* Add plugin dev documentation for webhook callbacks
* Fix tests
* Add note about namespacing webhook data
* 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
