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]

improvements to the command line interface for caching

Some StudentModule entries were written with newlines at the end of the
course_id. This script finds those entries within a given time range,
archive them, and merges records if necessary (capa problems).

clear request cache to avoid memory leak

update to use https

delete courses in the same transaction as writing them

python3 compatibility

* Renames discussion_module to discussion_xblock
* Moves common/lib/xmodule/xmodule_discussion to openedx/core/lib/xblock_builtin/xblock_discussion

This moves the functionality of the 'export_course'  management command from lms/djangoapps/courseware
over to the Studio codebase.  This reflects its use going forward to be run with cms settings, to
export the content of the Studio modulestore instead of the LMS modulestore.  The management
command is used by an analytics workflow to output course content for researchers.

add test case for discussion_id already set in XML

This is a clone (copy) of CSMH's declaration and methods with an added
id of UnsignedBigInAutoField

We should be able to delete the save_history code, but needs testing.

Add error logging when capa failures happen

Put StudentModuleHistory into its own database

Bump out the primary key on CSMHE

This gives us a gap to backfill as needed.
Since the new table's pk is an unsigned bigint, even for people who don't
consolidate CSMH into CSMHE, the lost rows are unlikely to matter.

Remove StudentModuleHistory cleaner

Also remove its corresponding ACCESS_REQUIRE_STAFF_FOR_COURSE feature
flag.

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

The old line:

    from path import path

produced pylint errors because of the baroque way that path.py defined
"path".  We tried to get them to change how they defined it, but they
deleted the name instead: https://github.com/jaraco/path.py/issues/102

(Jason then changed his mind, but this is a better way to use path.py,
it avoids the pylint error at least.)

Remove remove_input_state.py, as it was one-time fix code, and is not worth porting to the new interface

This required the following changes to the DjangoXBlockUserStateClient
semantics:

1) Changes get/get_many to return XBlockUserState tuples, rather
than state dictionaries or (block_key, state) tuples.
2) Raises DoesNotExist if get_history is called on an XBlock that has
had no data saved to it.
3) Returns XBlockUserState tuples as the results of get_history.

PLAT-484

While this may have served a purpose at one point, it's now been broken
for more than 2 years [1]; a critical code path makes a call to a
function which is no longer imported.

[1] commit 84cb0ce9
    Date:   Fri Dec 21 13:15:37 2012 -0500

Split modulestore throws duplicate errors if course locator is not unique.