Begin to replace OpenID with omniauth

Only UI yet is a link on the dedicated login page, a lot of OpenID remains are still in there and things are absolutely not secure or nice.
Omniauth: Allow correcting validation errors via registration form

This also sets User#password_change_allowed? to false for users
authenticated via Omniauth and thus hides login and passwords fields
from the register form when using Omniauth.
Omniauth: Hide authentication fields for users with external source

This removes identity_url from safe_attributes and permitted_params
and prevents admins from setting it as we are using identity_url now
to identify users with an external authentication provider.
Refactor UsersController#update a little
Omniauth: Fix lost_password deciding whether to allow reset
redirect to first_login page for all new users to set accessibility mode
Replace of flash notice for registration scenarios with automatic login
AccountController: Rename method
add omniauth failure callback and specs for omniauth login/registration
bugfix for register method and refactoring of specs

  Bugfix register method was missing user.save for exteranl
  authentication other than omniauth.

  Refactoring of omniauth specs to use let method to set omniauth
  hash.
Removal of unnecessary auth_hash method
Small refactoring of omniauth specs
PermittedParams: Fix comment about group_ids
Fix permitted_params_spec expecting identity_url to be assignable
Fix PermittedParams#user_register_via_omniauth

This fixes an example in the AccountController spec.
User update: Add missing admin and login fields to permitted params
User Create: Fix permitted params
User: Move auth_source_id filtering to PermittedParams
User: Make sure force_password_change can only be activated if allowed
Removal of trailing white spaces