Bug 1075473 - Allow is_staff users to see the cancell all button

This should show the cancel all jobs button on all trees if the user is logged in with an is_staff account, and show it for everyone on try group trees. 

I couldn't figure out how to conditionally change the appearance of the button for non-staff users on non-try trees. I wanted to do something like set "cursor: 'not-allowed'" and reject the click's action for that case, but I'm not sure I understand how ng-class works, or if there's some other way to do that.