Announcement Read Receipts

[ghost]

Currently all active announcements are shown to the user. While they can be dismissed, this is only held in local storage rather than being persisted. Announcements need to be deleted by mods to stop being pushed.

I believe we should defined a date range for announcements so that the client can pull announcements that are relevant. For example, the announcement for audio-only mode probably should have lasted only a month after deployment.

Authenticated users should be able to mark announcements as read and have this persist in the database.

I'm not sure if we want to provide a way to let the user see read announcements. I'm guessing most will just dismiss the announcement without much thought.