[openstreetmap/openstreetmap-website] Add notifcations functionality (Issue #3498)

[Sky]

### Description

It can be based on messages where if the sender_id is 0, it can be hidden from the messages inbox and shown in the notification's inbox. That way you won't have to add new tables to the DB. Notifications can be received for the same things that user's receive emails (notes, changeset comments, somebody friended them etc.).

### Screenshots

_No response_

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