CreateWiki: migrate away from exec

We should stop using shell scripts for creating users. https://github.com/Orain/ansible-playbook/issues/403 is the biggest CreateWiki issue going on, and while @JohnFLewis can try to use hhvm for this instead of the current "php" (https://github.com/Orain/CreateWiki/blob/master/SpecialCreateWiki.php#L135), I wonder why we use shell scripts. This can better be handled using SQL I guess.
