Send announcements to users

Instance-wide, announcements that are broadcast and shown to users, used by admins to show notifications about maintenance, or any news, etc. Would require integrating this in api plus in `crybapp/web`.

I feel we would need to do #88 first though, unless we want to make more WebSockets (no) or fetch an API endpoint every few (would prefer not to). This would also most likely require progress in the admin panel first, unless we make a script that just contacts the API endpoint to post announcements.