Delete notifications when the underlying content is deleted

[iambibhas]

If the underlying document is deleted right now after notifications have been created, there is an exception raised when users open the Updates tab. Instead, when a document is hard/soft deleted, all related notifications need to be deleted with it.

[jace]

This requirement applies to hard deletes only. With soft deletes, the object is in charge of revealing available content.

[jace]

Side-effect of #1007: if the notification was unread, it is not rendered, and therefore cannot be marked as read. The user will be permanently told that they have unread notifications, but won't be shown them.

This issue affected @anandvenkatanarayanan and was discovered when testing the implementation in #1052.