ldapBackend.py: Do user.save() after user-create's add-group

* When a user doesn't exist and needs to be populated from ldap, the
  user.save() is located inside the conditional code for setting
  LDAP_AUTH_GROUP. This commit moves it after that so the save() always
  happens (even if LDAP_AUTH_GROUP is set in settings.py it might bork
  before the save() if the group doesn't exist).

* Also moved it and the user.is_active assignment to after the
  "user-exists" try-except block, to be more DRY