This reverts commit 65f13dde.

This reverts commit 88dd85ca, reversing
changes made to bf8aef33.

This modifies the lang-pref django app to:
a) Use the current value of the 'edx-language-preference' cookie to set the
   users Accept-Language header on an incoming request.
b) At the end of the request, update the 'edx-language-preference' cookie
   to reflect the users current Language Preference choice, if any.

[LEARNER-542]

ECOM-4776

Removed the Darklang preview-lang and clear-lang parameters and added the new DarkLang settings form at /update_lang

Remove the DarkLang middleware from the LMS

Created and basic routing to the update_lang page for the GET Request
TNL-4742

Basic form functionality

Working example in LMS of the form to set the language

Login now required to change the preview language, and corrected some minor bugs

Updates to move the template code to lms and to correct minor defects
TNL-4742

Added template for preview_lang.html to cms
TNL-4742

Changed filename of darklang.py to api.py to match convention
TNL-4742

Updated and refactored the Darklang tests
TNL-4742

Updated comments in tests
TNL-4742

Formating updates
TNL-4742

Updated comments and formatting
TNL-4742

Corrected i18n tests and corrected PEP8 format issues
TNL-4742

Code Lint/PEP-8 corrections and upates
TNL-4742

Removed constant that was not needed (to be squashed)
TNL-4742

Added init method to clear up PEP8 Warnings (will squash)
TNL-4742

PEP-8/Lint issue resolved (squash)

Updated for i18n
TNL-4742

Refactored the preview_lang.html page to use a common included template

Refactoring and changes from PR comments
TNL-4742

Correction for safecommit violation (Squash)
TNL-4742

PR changes and refactoring (Squash)

Updates to reduce changes made in the urls used
TNL-4742

Removed unneeded aria-described by and bug in MAKO Template (squash)
TNK-4742

Updated docstring comments

Clarified form response text

Minor PR request (Squash)

Refactoring of how the responses are generated within the DarkLang views file

A series of refactors in response to PR comments

Method name change for clarity in reponse to PR comments (Squash)

Updates to tests per PR requests (Squash)

Minor comment updates for clarity and PR requests (Squash)

Updated per PR comments and added a test for empty preview_language

Layout and code style updates (Squash)

Updated test to contain method in the request.

Removed the Darklang preview-lang and clear-lang parameters and added the new DarkLang settings form at /update_lang

Refactored tests and added some tests for coverage, corrected defect with empty input codes

Removed unused and obsolete code

Corrected test errors, resolved PR comments, and updated comments to clarify testing
TNL-4742

Updated tests to deal with Pylint quality issue (Squash)

Updated tests to better reflect test case and PR updates (Squash)

This abstract class contains most of the fields (aside from the id and
foreign key to StudentModule that the subclasses need to manage).  It
also provides a get_history method that abstracts searching across
multiple backends.

Move router code to openedx/core
We need to use it from cms and lms.
Ensure aws_migrate can be used for migrating both the lms and cms.

Handle queries directed to student_module_history vs default and the
extra queries generated by Django 1.8 (SAVEPOINTS, etc).

Additionally, flag testing classes as multi_db so that Django will
flush the non-default database between unit tests.

Further decouple the foreignkey relation between csm and csmhe

When calling StudentModule().delete() Django will try to delete CSMHE
objects, but naively does so in the database, not by consulting the
database router.

Instead, we disable django cascading deletes and listen for post_delete
signals and clean up CSMHE by hand.

Add feature flags for CSMHE
One to turn it on/off so we can control the deploy.
The other will control whether or not we read from two database tables
or one when searching.

Update tests to explicitly use this get_history method rather than
looking directly into StudentModuleHistory or
StudentModuleHistoryExtended.

Inform lettuce to avoid the coursewarehistoryextended app

Otherwise it fails when it can't find features/ in that app.

Add Pg support, this is not tested automatically.

Tests would have caught issues raised in LOC-87

This reverts commit 09425c3d, reversing
changes made to 410b9282.

The old code would compute the language direction once when the template
was loaded.

TNL-2269

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]

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