Major authentication provider refactoring to support

- Modular and easily configurable messages with common context
- Fine grained provider policies to support appling specific policies to
  users and/or groups

Key points:

- Use auth_providers.AuthProvider instances where auth provider logic is
  needed. Instances get properly initialized with the available context
  (with no user/signup view, with user/login view, with user and
  identifier/profile view).

- All authentication provider messages are now accessed using the
  get_*_msg AuthProvider attributes.

- Provider policies logic is handled from  get_*_policy attributes.

- All provider messages may be overridden globally or per provider level from
  settings::

  # global change
  ASTAKOS_AUTH_PROVIDER_NOT_ACTIVE = 'Provider not active'

  # change only applies to shibboleth provider
  ASTAKOS_AUTH_PROVIDER_SHIBBOLETH_NOT_ACTIVE = 'Shibboleth is not  active'

- Provider policies may be overridden in settings::

  # ALL users wont be able to add shibboleth login method from their
  # profile
  AUTH_PROVIDER_SHIBBOLETH_ADD_POLICY = False

- New provider policies profile model added. Profiles can be assigned to
  a group or/and a specific user.

- All tests updated to match the auth providers changes.

- New management commands included

  * user-auth-policy-{add, list, remove, set, show}
    Manage authentication provider policy profiles.

  * user-group-{add, list}
    User group management commands

- Updated user-list to optionally display auth provider information