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]

Add consent check to course access prerequisites; add utility functions to provide interface to course-specific consent in Enterprise app

This reverts commit 8c009881, reversing
changes made to 5b6e2dd5.

nit: css

TNL-5990

This reverts commit d59ab18b, reversing
changes made to 0ebab35e.

Initial commit

TNL-5484

TNL-5835

As part of the Robust Grades rollout, we expect to see some DatabaseErrors in
various methods that write to the database. Since the success of this write
operation is not needed for the end-user, we just log and swallow the error.

In the future, we'll also enqueue an async task to finish the write operation
that failed.

Discussion is restricted to the CCX course (there is no sharing of
discussions with the parent course or other CCX instances).

Adds ENABLE_SUBSECTION_GRADES_SAVED feature flag to both lms and cms. Also
installs the wiring that will allow robust grades to be used for courses
that enable it. This functionality is still gated by the feature flag
and should not be used until the remaining robust grades work is finished.

TNL-5002

Converts the dates on the dashboard, sidebar navigation, and important course dates to user specified time zone.

Performance improvement, this cache is no longer needed thanks
to block_structure caching data.

TNL-4874

1. update query counts with more accuracy
2. don't use defaultdict in block_structure

By default, disable all caching in tests, to preserve test independence.
In order to enable caching, inherit from CacheSetupMixin, and specify
which cache configuration is needed.

[EV-32]

MA-2258