[3.0] Move Room from Chronos to Core

We should move the Room model from Chronos to Core, in order to enable applications to use it even if not working with weekly timetables.

One app that would benefit from this is Plank (the inventory app).

ToDo:

• Migrations
  • Core until create_room
    • (1) Create Model room
    • (2) core: Copy Data (IDs) [depends on: 1]
    • Migration after create_room optional dependency on chronos delete_room
  • Chronos until delete_room
    • Migration before delete_room mandatory dependency on core create_room
  • (4) strolindling: Alter Relation [depends on: 1]
• Fix imports in other apps. (chronos.model.Room -> core.model.Room)
  • Make sure that there is a migration that also changes the relations in all apps that then have a changed Import.
• Update permissions, rules and predicates

The migrations must run in this order.