global: integrate changes from invenio-saml

[slint]

From https://github.com/inveniosoftware/invenio-saml/tree/master/invenio_saml/invenio_accounts the different files/functions have to be moved in the equivalent files in this module:

• models.py: defines UserIdentity, and makes sure that it doesn't clash with invenio_oauthclient.models.UserIdentity
  • This requires performing the same compatibility "trick" to import UserIdentity correctly
• utils.py + errors.py: move all external identifiers helpers functions, and make a list of places that can re-use these functions, invenio_oauthclient/contrib/*.py, invenio_oauthclient/handlers.py`