Integration tests for omniauth authentication

  The specs use the developer omniauth strategy
  provided by omniauth itself to perform the tests.
User edit: Show external authentication provider
Add two routes for MyController (no longer use default route)
AccountController spec: Use route helpers for MyController
Clear identity_url on existing users and back them up
Add missing german translations related to omniauth
Enhancement of integration tests for omniauth authentication

  Includes specs for register a user on the fly.
Bugfix for back url if registration is performed via omniauth
Removal of unnecessary CGI.unescape for back url
Add spec for User#uses_external_authentication?
Add spec for User#change_password_allowed?; migrate testunit test
Add spec for AccountController#identity_url
unifies creation/deletion/error notices in user's membership tab
Add deletion notice
Add update notice
Fix code layout
Fix flash message activation
do not display successful flash message when in wasn't
re-render everything so there won't be multiple error flash messages on the page
Precompile acitvate error messages js