Add basic user search

@houndci @cgruppioni @georgetmayorga
You can use a form_tag when you're not acting directly on an instance of an active record object and are going through a different path than expected. 

A ```form_for @var``` is going to make the assumptions on the object and the path, whereas in ```form_tag``` you supply it with the path, the method it uses, and then the rest of the form you supply it with params.