Fix visual/push notifications for edited messages

[alya]

As discussed in CZO thread and #26166 (comment), we should fix the following visual/push notification quirks for edited messages:

• Editing a message does not re-rerender either desktop or mobile push notifications that are currently visible.
• Editing a message to remove a mention does not revoke desktop or mobile push notifications either.
• Editing a message to delete all its content works the same as any other edit.

We should also update /help/edit-a-message.md#message-notifications once we've fixed the above.

Note that this issue used to be part of #26584. We're splitting it out as this part is much more complex.