Version 0.9.0 - twitter, rails 4 and deprecations

This release features a great contribution from the Rails Girls Summer of Code:
The landing page of the webapp can now include a twitter feed to display
news from the provider.

Other than that this is a maintainance and transition release.

* Twitter feed on main page (thanks theaamanda and lilaluca).
* upgrade to rails 4.2
* upgrade to bootstrap 3

Upgrading:

* We now use rails 4's `secret_key_base`. Please make sure to supply it
  in config/config.yml for production environments. If you are using the
  leap platform that will already take care of it.

Deprecations:

* We have not seen any active use of the **billing** functionality.
  So we deprecate it and will probably drop it in one of the next releases.
* We will replace the user facing **help desk** functionality with a single
  sign on mechanism to integrate with other help desk systems.
  We will maintain the endpoint to submit tickets and the ticket management
  in the admin interface. That way it should also be easy to create your own
  ticket submission form.
* We deprecate the ability to **signup and login** directly through the webapp.
  We will remove it in the future for security reasons. Signup and Login should
  only happen through bitmask to prevent password phishing and js injections.