search term for users and groups may occur anywhere in the name or displ...

...ayname, not just in the beginning.

Solves #8959 

How to test? Have a local user with displayname "John Smith". On the users page, filter for "smith".
Before: the user will not be shown.
Now: he will.

Please test/review @icewind1991 @raghunayyar @Xenopathic 

cc @MTRichards 
This function doesn't exist?