PUSH Messaging support draft for android

Added the initial push support for android.
The iOS support will follow soon.

- Added a Messages section
  2 views, Messages and Message
  1 store, Messages
  1 model, Message
  1 controller, Push
- Added a Messages button in the AppBar.
  The badge of the button is used as the notification for the
  number of unread messages.
- Bugfix: vertical scroll for the Mensa view (small screens issue)
- Bugfix: better description in the Mensa view