Refactor test DB helpers with insertEntity
Reorder imports, include ident in created User
Reorder imports, hlint, add failing Unsubscribe spec
Clean database before and after specs
Implement unsubscribe filtering
Refactor and implement unsubscribing
Add more end-to-end spec on notifications
Use an unsubscribe key, rename module

- Using a key means Unsubscribe can be used for any kind of notification, not
  only one tied to article/thread
- Modules should be singular as per johan-tibell style guide