REGISTRAR: fill user to session if possible

- Fixed case when user join identity.

- Retrieval of form must always check, if user is known by ext source,
  and if yes, fill user in session.

- Creation of application must do the same so user is set to application.
  It would be paired on approval, but it's nicer for admins to see username
  rather than actor/extSource.
Merge pull request #540 from zlamalp/regAuthz

REGISTRAR: fill user to session if possible