fixes possibility of a user password change being sent out when a password couldn't be/wasn't changed

previously it never returned any users

This integrates the functionality of the now outdated addomain plugin
directly into the authad plugin and makes multi-domain setups usable
without SSO.

See also https://github.com/cosmocode/dokuwiki-plugin-addomain

As suggested by @selfthinker

In Novell eDir the group search returns strings, not arrays. Added if-statement which determines if the result is an array or an string.

fix bug that only allows admins any AJAX calls