Implement global announcements

[krishnans2006]

It would be nice to add global announcements to Tin.

One such implementation would look similar to tjcsl/othello-tourney, with message configuration in the settings and display logic in templates like this and this.

However, it may make more sense for the feature to be implemented as an app/model. This allows for further feature expansion, with ideas like course-specific announcements and "mark as read" functionality, similar to tjcsl/ion's announcements feature.