Hopefully last changes to get tests running in Django 1.11

Keeping this branch rebased to track changes to tests and check for
regressions.

cms/envs/common.py

@@ -1194,7 +1194,6 @@ MAX_FAILED_LOGIN_ATTEMPTS_LOCKOUT_PERIOD_SECS = 15 * 60
 # The order of INSTALLED_APPS matters, so this tuple is the app name and the item in INSTALLED_APPS
 # that this app should be inserted *before*. A None here means it should be appended to the list.
 OPTIONAL_APPS = (
-    ('mentoring', None),
     ('problem_builder', 'openedx.core.djangoapps.content.course_overviews.apps.CourseOverviewsConfig'),
     ('edx_sga', None),
 

lms/envs/common.py

@@ -2878,7 +2878,6 @@ ALL_LANGUAGES = [
 # The order of INSTALLED_APPS matters, so this tuple is the app name and the item in INSTALLED_APPS
 # that this app should be inserted *before*. A None here means it should be appended to the list.
 OPTIONAL_APPS = [
-    ('mentoring', None),
     ('problem_builder', 'openedx.core.djangoapps.content.course_overviews.apps.CourseOverviewsConfig'),
     ('edx_sga', None),