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]

We need to be able to track which site a given user account was created on. This
change will create a UserAttribute model with a key of 'created_on_site' and a value
containing the domain of the site on which the user accounted was created.

WL-977

This reverts commit bc418c47.

ECOM-6463

This commit contains back end changes necessary to load programs from the catalog in all contexts. The existing program munging utility is applied as late as possible to avoid conflating this work with changes to the front end; those will be made separately.

ECOM-4422

We should not be using custom signing keys for each service at this time. We may want to return to this strategy in the future; but, this is not the direction any of our other services are going in.

ECOM-6541

The user info is now updated every time a user loads the learner dashboard. Given that this is the page most learners land on after enrolling in a course, updating this cookie here will ensure that the enrollment status hash is relatively up-to-date.

ECOM-4895

This will allow the marketing site to determine if locally-stored enrollment data needs to be refreshed.

ECOM-4896

Will fall back to stored certificate grade if needed as we've discussed.

TNL-5314

This specifically enables/disables the underlying comment service client
used to make calls to the service.  When disabled, this client will now
throw an exception which can be propagated upwards so that callers can
make the right decision about how to notify users of the error, or
handle retry, etc etc.

ECOM-5968

Show verification expiration message on learner dashboard and allow them to reverify if expiration is X days away.
ECOM-2979

Add UTM cookie tracking for registrations.
Add config model for cookie names.

ECOM-5539

Move oauth_dispatch from lms to openedx/core.

ECOM-4641

This reverts commit fe136122.

Celery need to serialize the arguments for a task. Here, user is not serializable,replace it with ID. ECOM-5784

It should help catch unicode encoding problems earlier.

This also updates python-saml dependency to a version that includes a
patch for unicode strings.