Remove duplicated course listings template code on the student dashboard.

Original Commit Messages:

use edx's own get_parent method, rather than our own.

add field to unique constraint to avoid MultipleObjectsReturned in case of multiple browser clicks on submit

fix 0011 migration, inherit from TimeStampedField and add composite index (migration only)

fix bug where adding an already registered user to a ccx would cause a crash due to an undefined variable

add assertNumQueries tests to test modules where override field providers are used

remove unnecessary teardown

implement recommended style for checking empty list

import utility methods rather than use duplicate code

added comment explaining date conversion to string for json

add logging for invalid users or emails when enrolling students

add comment about xmodule user state

avoid using get_or_create, which seems to be causing a race condition on schedule change save

relocate badly placed edvent handlers to fix multiple submit problem

individual students, and a reimplementation of the individual due date
feature.

This work introduces an architecture, used with the 'authored_data'
portion of LmsFieldData, which allows arbitrary field overrides to be
made for fields that are part of the course content or settings (Mongo
data).  The basic architecture is extensible by means of writing and
configuring arbitrary field override providers.

One concrete implementation of a field override provider is provided
which allows for overrides to be for individual students.  This provider
is then used as a basis for reimplementing the individual due date
extensions feature as a proof of concept for the design.

One can imagine writing override providers that provide overrides based
on a student's membership in a cohort or other similar idea.  This work
is being done, in fact, to pave the way for the Personal Online Courses
feature being developed by MIT, which will use an override provider very
much long those lines.

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]

Co-Authored-By: Christina Roberts <christina@edx.org>
Co-Authored-By: Daniel Friedman <dfriedman@edx.org>
Co-Authored-By: Don Mitchell <dmitchell@edx.org>

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

TE-489

The IDDE implementation relied on a StudentModule being created for a
particular block in order to set the extended due date on that block.
Since StudentModules seem to be created on demand whenever data is
written to an attribute with Scope.user_state, it meant that if a
homework problem hadn't yet been touched by a student it was possible
that the due date extension wouldn't take effect for that problem, even
if the due date extension was successfully set for the parent unit.

This patch fixes this problem by creating new StudentModules as
necessary in order to make sure extended due dates propogate properly to
all problems in a unit.

[LMS-2757]

This commit adds the non-courseware lms/djangoapps and lms/lib.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

Adds a feature to the edX platform which allows instructors to set
individual due dates for students on particular coursework. This code is
meant primarily for on-campus use--it is not intended that this feature
would be used for MOOCs. It adds a new tab, "Extensions", to the beta
instructor dashboard which allows changing due dates per student. This
feature is enabled by setting FEATURES['INDIVIDUAL_DUE_DATES'] = True.