Changes instances of User to ESPUser

In program.models.class_.ClassSubject.conflicts(), the teacher argument
is cast from User to ESPUser, and saved in the user variable. However,
there were still some instances of teacher being used in the method.
This commit changes those references to teacher to references to user.
Fixes bug in TeacherReviewApps module

Adds a missing aux_call decorator to TeacherReviewApps.review_students
Convert ClassSection.get_meeting_times() to use caching API

The existing function used local memory storage for the cached result
of self.meeting_times.all(), so in rare cases the result of
get_meeting_times() was non-empty when in reality the real list of
meeting_times was empty.  This caused errors in
ClassSection.initial_rooms().  Fixed by adding @cache_function and
depend_on_m2m.
(cherry picked from commit a51ddb2ad25568b0a9267407906c2dbe9c952da1)
Add regression test for meeting_times caching bug
Merge branch 'stable-jan12-bugfixes' of github.com:learning-unlimited/ESP-Website into stable-jan12-bugfixes
Change User to ESPUser in AdminClass coteachers view
Merge pull request #271 from learning-unlimited/stable-jan12-bugfixes

Remove instances of User; Add missing aux_call
Merge remote branch 'origin/stable-jan12' into mit_spark_2012