Feature: Site announcements

[jschuur]

Goal: A basic way to show updated to users when they log in. Major new features, site bugs etc.

• [x] show announcements above the user list
• [ ] announcements can be made without a site redeploy (i.e. come out of a database table, not the deployed site source)
• [x] users can close the announcement and they won't see it again, even if they restart the session
• [ ] make new announcements turn up for currently logged in users without a refresh

Keep this a small scoped feature without the need to e.g. show multiple announcements at once. Just always show the latest one, unless the user has seen/closed it.

[jschuur]

Initial version deployed in 3151d94d635dece1fec0818c949d9fe5e893e7aa, but still pulls data out of a git commit