Improve admin message seen/dismiss state. (#26223)

Improve admin message seen/dismiss state. (#26223)

Fixes #26211

Admin messages now have separate "seen" and "dismissed" fields. The idea is that an admin should be able to tell whether a user pressed the "dismiss for now" button. Instead of using "seen" as "show this message to players when they join", "dismissed" is now used for this.

Existing notes in the database will automatically be marked as dismissed on migration. A note cannot be dismissed without being seen (enforced via constraint in the database too, aren't I fancy).

As part of this, it has become impossible for a player to play without dismissing the message in some form. Instead of a shitty popup window, the popup is now a fullscreen overlay that blocks clicks behind it, making the game unplayable. Also, if a user somehow has multiple messages they will be combined into one popup.

Also I had enough respect for the codebase to make it look better and clean up the code somewhat. Yippee.