There are a number of Django Signals that are on the modulestore's
SignalHandler class, such as SignalHandler.course_published. These
signals can trigger very expensive processes to occur, such as course
overview or block structures generation. Most of the time, the test
author doesn't care about these side-effects.

This commit does a few things:

* Converts the signals on SignalHandler to be instances of a new
  SwitchedSignal class, that allows signal sending to be disabled.

* Creates a SignalIsolationMixin helper similar in spirit to the
  CacheIsolationMixin, and adds it to the ModuleStoreIsolationMixin
  (and thus to ModuleStoreTestCase and SharedModuleStoreTestCase).

* Converts our various tests to use this new mechanism. In some cases,
  this means adjusting query counts downwards because they no longer
  have to account for publishing listener actions.

Modulestore generated signals are now muted by default during test runs.
Calls to send() them will result in no-ops. You can choose to enable
specific signals for a given subclass of ModuleStoreTestCase or
SharedModuleStoreTestCase by specifying an ENABLED_SIGNALS class
attribute, like the following example:

    from xmodule.modulestore.tests.django_utils import ModuleStoreTestCase

    class MyPublishTestCase(ModuleStoreTestCase):
        ENABLED_SIGNALS = ['course_published', 'pre_publish']

You should take great care when disabling signals outside of a
ModuleStoreTestCase or SharedModuleStoreTestCase, since they can leak
out into other tests. Be sure to always clean up, and never disable
signals outside of testing. Because signals are essentially process
globals, it can have a lot of unpleasant side-effects if we start
mucking around with them during live requests.

Overall, this change has cut the total test execution time for
edx-platform by a bit over a third, though we still spend a lot in
pre-test setup during our test builds.

[PERF-413]

TNL-6331

Hackathon XV project

for TNL-6214

Also fixes issue where ORA grades were never updated, by including
them in this check.

TNL-5994
TNL-5995

for TNL-5045

Using the guideline "Course staff should have access to features related to
individual learners, but not course-wide modifications", most tools on the
student admin sub-tab of the instructor dashboard should be available to
staff as well as instructors.

TNL-5907

Remove deprecated SingleSectionGrader.  TNL-5987
Remove display_name and module_id from Scores objects
Update CourseGradeFactory.__init__ to not be user-specific
Update some callers to use CourseGrade class instead of "summary" dict
Remove no longer needed course_grades.py module.
Renamed django signal from GRADES_UPDATED to COURSE_GRADE_CHANGED

For TNL-5993

* Centralize creation of quick request objects.
* Isolate caches to individual tests to prevent test ordering
  dependencies.

TNL-5811

Makes use of the new SUBSECTION_SCORE_CHANGED signal to trigger a task that
updates persisted course grade values. We've also renamed SCORE_CHANGED to
PROBLEM_SCORE_CHANGED to head off any issues with unclear signal names.

TNL-5740

TNL-5046

For better user-facing performance, the SCORE_CHANGED signal is now handled by
enqueueing an async task to update the relevant stored grade, rather than
making the request wait until that operation finishes.

TNL-5738

This reverts commit 9aa35cd8.

Some XModules are breaking their contracts and defining has_score but not
max_score.

TNL-5715

fixes signal handling for delete student state

This reverts commit 0ea8e1b9, reversing
changes made to e3db08f1.

Potential Grades revert PR

This reverts commit e3db08f1, reversing
changes made to 51c2444d.

The interface for custom management commands changed in Django 1.8 which
caused the `compute_grades` command to stop working. In order to fix it
I added an `add_arguments` method for specifying the accepted arguments
and updated the command to allow for passing a list of courses to run
the `offline_grade_calculation` against.

This management command has been broken since before the django 1.8
upgrade, so I'm deleting it and all its tests, and updating a few
other files that used these methods but are easily refactored away.

Basic architecture of Maintenance App - SUST-19, SUST-42. Implement Force Publish Course view. SUST-46