Closes #15621: User notifications (#16800)

* 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

docs/models/extras/notification.md

@@ -0,0 +1,17 @@
+# Notification
+
+A notification alerts a user that a specific action has taken place in NetBox, such as an object being modified or a background job completing. A notification may be generated via a user's [subscription](./subscription.md) to a particular object, or by an event rule targeting a [notification group](./notificationgroup.md) of which the user is a member.
+
+## Fields
+
+### User
+
+The recipient of the notification.
+
+### Object
+
+The object to which the notification relates.
+
+### Event Type
+
+The type of event indicated by the notification.

docs/models/extras/notificationgroup.md

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+# Notification Group
+
+A set of NetBox users and/or groups of users identified as recipients for certain [notifications](./notification.md).
+
+## Fields
+
+### Name
+
+The name of the notification group.
+
+### Users
+
+One or more users directly designated as members of the notification group.
+
+### Groups
+
+All users of any selected groups are considered as members of the notification group.

docs/models/extras/subscription.md

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+# Subscription
+
+A record indicating that a user is to be notified of any changes to a particular NetBox object. A notification maps exactly one user to exactly one object.
+
+When an object to which a user is subscribed changes, a [notification](./notification.md) is generated for the user.
+
+## Fields
+
+### User
+
+The subscribed user.
+
+### Object
+
+The object to which the user is subscribed.