Updates to Educator and Grader permissions

This should resolve issues #275 and #271.

All uses of `Klass.is_educator?(user)` were replaced with `Klass.is_teacher?(user)`, a new method which returns true only if the user is an Instructor or a TeachingAssistant.  This should preserve all existing behaviors since there were no Graders in the system at the time of the change.

Uses of `is_teacher?(user)` were changed as needed by Graders to do their job, thus limiting the scope of their powers to the absolute minimum (hopefully).

Another minor change was to allow Educators to see Assignment labels when the associated Class is not part of a controlled experiment.