Generalise announcements

From the intermediate dev meeting with @hansegucker yesterday:

The system currently called "hints" shall be renamed to announcements and turned into a genric mechanisms for apps to register announcements that are linked to a time span and a set of recipients (recipients can be users or groups, or probably other kinds of objects — implementation detail: everything that either is a Person or has a relation on Person).

Announcements are displayed on the dashboard, but can be pulled in by other apps, like Chronos to render them in the substitution plan or by the upcoming digital signage app. Adding announcements also triggers notifications to the users related to the recipient objects (cf. #165 (closed)).