Not mensa-specific

The dev sprint found out that this app is not specific to mensae, so:

• Conclude that this app managed time-based documents

• Reword all parts of the app so this is reflected
• Restructure the data model, e.g. like so:
  • DocumentGroup defines a set of documents that have a certain validity/schedule
  • Document defines one uploaded file in this group
• Find a new name that represents that → Resint
• Implement permissions