Sanitize user auto creation in CentralAuth

Minor changes:
* Avoid double calls to functions
* Use the respecting User class function rather than a Title
  one to sanitize user names.

Most important change is that we use CentralAuthUser::getInstance
on the initial User object to make sure it is already set and sane.
With that change I hope to strangle bug 39996 silently.

Bug: 39996
Change-Id: Ie87319134d92fe6ecb9cf46cf3aba408724bd361
Merge "Sanitize user auto creation in CentralAuth"