Fixed identification of users with extern auth provider (LDAP)

Created by: jirutka

We cannot use email as identification of user when using LDAP. When user decides to change his e-mail address in Gitlab (but not in LDAP), he loses his account and new one with the new e-mail address is created.

I added new user attributes extern_uid and provider and changed User.find_for_ldap_auth() to find user by these attributes instead of email.