MiqWidget.available_for_user should check by both role and group.

MiqWidget visibility can be set either by role or by group. 
Currently MiqWidget.available_for_user checks only by role. So widget visibility does not work for group.

https://bugzilla.redhat.com/show_bug.cgi?id=1158636
@Fryguy A widget's resource should be one of MiqReport, RssFeed or null. 
So the checking of w.resource == user is not right.
Do you think I should delete it in this PR or a separate one?