Some low hanging fruit optimization of forums tests.

Adds a read-only mode to the discussion module which disables any
controls for updating the discussion (votes, follows, replies,
etc). This mode is enabled for users who are not a member of the team,
and are also not moderators, admins, or community TAs.

TNL-1908

* Handle grading errors

TNL-1256

course cohort settings only once.

TNL-1258

TNL-1258

Update edx-lint to the version that checks if tearDown uses super.

Convert a number of tearDown methods into addCleanup.

Remove some unneeded tearDown methods: no need to call patch.stopall if
none of them were started with patch.start.

I can find no other record of this class.

TNL-650

Conflicts:
	lms/djangoapps/django_comment_client/base/views.py
	lms/djangoapps/django_comment_client/tests/test_utils.py
	lms/djangoapps/django_comment_client/tests/utils.py
	lms/djangoapps/django_comment_client/utils.py

This reverts commit 086a2ba7, reversing
changes made to a44132ec.  Some minor
code quality issues were addressed as part of this change.

Conflicts:
	lms/djangoapps/django_comment_client/base/views.py
	lms/djangoapps/django_comment_client/utils.py

When two or more instances of Discussion XBlock were configured
with the same discussion target (Category/Subcategory),
only one of the blocks would be shown on the Course Discussion page.
This was the source of several bugs when trying to edit discussion threads.

This patch adds incrementing numbers to the title of each duplicate
subcategory when rendering the Course Discussion to make sure that
all of the threads are visible in Course Discussion.

TNL-650

I can't seem to find any references to this code outside of the library
and the tests themselves.

The existing pattern of using `override_settings(MODULESTORE=...)` prevented
us from having more than one layer of subclassing in modulestore tests.

In a structure like:

    @override_settings(MODULESTORE=store_a)
    class BaseTestCase(ModuleStoreTestCase):
        def setUp(self):
            # use store

    @override_settings(MODULESTORE=store_b)
    class ChildTestCase(BaseTestCase):
        def setUp(self):
            # use store

In this case, the store actions performed in `BaseTestCase` on behalf of
`ChildTestCase` would still use `store_a`, even though the `ChildTestCase`
had specified to use `store_b`. This is because the `override_settings`
decorator would be the innermost wrapper around the `BaseTestCase.setUp` method,
no matter what `ChildTestCase` does.

To remedy this, we move the call to `override_settings` into the
`ModuleStoreTestCase.setUp` method, and use a cleanup to remove the override.
Subclasses can just defined the `MODULESTORE` class attribute to specify which
modulestore to use _for the entire `setUp` chain_.

[PLAT-419]

TNL-670

JIRA: TNL-710

IMPORTANT: this commit converts the course_groups
package to using migrations.  When deploying to an
existing openedx instance, migration 0001 may fail
with an error indicating that the CourseUserGroup
table already exists.  If this happens, running
the 0001 migration first, with the --fake option,
is recommended.  After performing this step,
remaining migrations should work as expected.

Move modulestore config for tests to an importable location
Disable pylnt warning for lms imports in common tests
Refactor all testcases that loaded all xml courses
TE-610
TE-489

TNL-24