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.

This will allow users to delegate permissions to a 3rd party service.

TNL-3137

TNL-3137

TNL-3137

TNL-3137

TNL-3137

TNL-3137

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.)

TNL-2413

TNL-2413

TNL-2487

Fix n-click behaviour on poster.
Fix unit tests.
Fix handler for non_en lang for bumper.
Add more tests.
Fix docstrings.
Fix pep8.
Fix static redirection with bumper.
Fix button in IE11.
Add video_bumper field in bok_choy.

Fix pylink violations.

Update docstrings and some clean up.

Rename edx_video_id in bumper tests.

Fix too long lines in help text.

Address ui comments.

Fix bumper events.

Refactor bumper-transcripts code, fix bugs, address comments.
Squashed commits:
Fix download transcript button.
[74e0c8c] Fix quality
[a759f33] Fix error, when sub contains extension.
[b30755c] Revert "Add video files to host for transcripts."

This reverts commit cf8a96bf84346e17b6ad57ad4cc6a27d7a9118cd.
[36f038a] Add video files to host for transcripts.
[23f1655] Fix pep8 and pyling issues.
[0f1f9d2] Update acceptance test.
[765a27d] Wait for ajax in captions.
[8ae72a3] Fix logic.
[063450f] Fix unit tests.
[d1075fc] Fix handlers tests.
[25d31ad] Update bumper_utils.
[cb5f9df] Remove maxDiff.
[8738b1a] Code cleanup.
[87dbcb7] Fix issues with transcripts.
[ec899de] Fix transcripts in serializers.
[444b1fc] Fix transcripts typo.
[d524cb5] Fix bumper.
[f62cf22] Fix video mongo tests.
[8f1b55a] Fix dispatches.
[53bc308] Add more fixes.
[d5e3723] Fix test_video_handlers and rename the method.
[93efc23] Fix mobile tests.
[740e2ae] Fix pep8 and pylint.
[47cfb66] Address comments, add fixes.
[4e499d9] Add fixes.
[8353553] Add improvements.

Updated dispatch values)
.

Use ddt in bumper handler tests.

Move common metadata to single place.

Fix style.

Update docstring.

Fix poster button.

Improve bumper events.

Fix test after rebase.

Address comments.

Download transcript: use def video lang, not bump.

Renamed date_last_view_bumper to bumper_last_view_date.

Rename do_not_show_again_bumper to bumper_...

Address comments.

Fix tests for download for en lang.

Fix bumper logic.

Update strings.

Update resizer.

Remove resizer.

Fix unit tests.

Add tests.

Fix bumper events.

Clean up tests.

Fix pylint violations.

Fix pep8 and pylint violations.

Update docs and method names.

Update events.

Make /static/ prefix a must.

Fix wrong code.

This allows course authors to choose between two difference licenses for their
course content: All Rights Reserved, or Creative Commons. In the backend, XBlocks
that wish to allow custom licenses need only inherit from LicenseMixin, which
adds a `license` field as a string.

License information is displayed in the Studio editor view, and just below the
rendered XBlock in the LMS. In addition, if the course block itself has a custom
license set, this license will display just below the main body of the page
on courseware pages.

This entire feature is gated behind the LICENSING feature flag.

TNL-1924

TNL-1538

path.py objects report themselves as class objects, which confuses the heck out of
pylint. It tries to match variable names using the class-rgx regular expression
instead of the constant-rgx regular expression, and it doesn't match, so it
throws an error. Not sure how to fix pylint, so we'll just ignore these errors.