reminder for user's cats/reported cats

[DerGernTod]

if a cat is still status unresolved, a owner/reporter should get a notification mail after x weeks (configurable)

since we don't really have members anymore but directly send unsubscribe/change emails to recipients, we can introduce a "Notification" class that is linked to a cat. whenever a cat is created, a notification for it is created, which stores a timestamp 2 weeks in the future as well as a modification token. these notifications get triggered once a day (all notifications with timestamp < now are triggered and their timestamp is updated to +2 weeks).

the notification should contain a link to a page where it can be configured to be longer.

todo:

• [x] remove "Member" concept from cats (implications!)
• [x] controller for notification modification
• [x] notification class containing
  • timestamp
  • edit token
  • linked cat
• [x] task to trigger notifications still missing:
• [ ] actual email sending