[openstreetmap/openstreetmap-website] Implement a reliable system to @-mention users in comments (Issue #6635)

[Pablo Brasero]

pablobm created an issue (openstreetmap/openstreetmap-website#6635)

### Problem

(Taken from https://github.com/openstreetmap/openstreetmap-website/pull/6612#issuecomment-3670633084)

Currently, users can mention other users by username using an "@-mention" in comments, etc. This creates a link to the mentioned user, and potentially sends a notification message (not sure this actually happens currently, but it's planned). When this happens, the username is stored as part of the comment text as entered, verbatim.

The problem is that users can change username at any time, making pre-existing @-mentions to them misleading. These mentions are left linking to non-existent users, or even to the wrong user if in the future someone else renames to a vacated username. In addition, we are aware that this has been used maliciously in the past.

### Description

Implement a selector of the sort you see in other apps (eg: GitHub, Slack), where you get a dropdown the moment you enter an "@" character, and then this gets stored as `@[user_id:$USER_ID]` (or something to that effect) that is interpreted and replaced as appropriate when renderding.

This implementation would also allow referring to users with spaces in their names, closing https://github.com/openstreetmap/openstreetmap-website/issues/6614

### Screenshots

_No response_

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