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]

The RLIMIT_NPROC limit is per-user so simultaneous student submissions
pile up and overflow the limit.  Setting this environment variable will
prevent spawning any threads (the 1 means only have one threads, the
already existing main thread).

TNL-6553

TNL-6553

Fix asides to copy them during publish operation (bug in modulestore/split_mongo/split.py:SplitMongoModuleStore._copy_subdag method)

TNL-6357

With 2.0.1, and setuptools 34.0.2, errors happen like:

File "/edx/app/edxapp/venvs/edxapp-sandbox/local/lib/python2.7/site-packages/packaging/requirements.py", line 59, in <module>
    MARKER_EXPR = originalTextFor(MARKER_EXPR())("marker")
TypeError: __call__() takes exactly 2 arguments (1 given)

(cherry picked from commit 73519b35)

This reverts commit c7d2d601, reversing
changes made to fb2173f4.

TNL-5619: removes enable/disable all button methods to save button state and disable active, modifies tests for new functions

[courseware] generate sequence-specific titles server-side (AC-695)

[courseware] generate sequence-specific titles server-side (AC-695)

case for empty subsections [AC-695]

move title logic into courseware context method [AC-695]

pep8

coffeescript -> js

js mistake

jslint

refactor sequence title generation slightly

missed a line

line too long

python is not javascript

ugh js

ugh js part 2

This reverts commit 8b8cf5f9, reversing
changes made to 2716ffbf.

Hijacks the existing hide_after_due field to be repected as "hide after course
end" for self-paced courses. Includes tests.

TNL-6108

ECOM-4422

Also remove unused empty jsinput_css.css.

I missed these files the first time around.

TNL-5893

Increased number of retries, as it is expected that sometimes
a seed will be reused. TNL-6041