initial commit for a mixed module store which can interoperate with both XML and Mongo module stores

Contentstore views pylint fixes

Workaround for issue

Better wording of commit

Workaround for issue

Better wording of commit

Fixed spacing

capa optioninput template was missing msg output (needed for hints)

Make split mongo read-only API consistent with other modulestores.

Add test for invalid YouTube IDs strings

Also clarifies the contraction of location.course_id by throwing an exception for lcoations that are not of category course.

Add test for course_id method.

Fix/vik/oe issues

Fix error seen in combinedopenended modules

Move the silencing of numpy's warnings into test_calc.py

..because that is where it is the most annoying/visible. Otherwise it really
has no effect on the LMS or anything else.

catch error in delete_course management script when group does not exist

Add filter for playback rates.

turn off i18n

Fix error seen in combinedopenended modules

The enrollment_action value enroll should not be translated.

Add Course Modes to LMS

adding django-admin commands to dump grades to a csv file

Add mode and is_active to CourseEnrollment, shift enrollment logic to model

Features coming down the pipe will want to be able to:
* Refer to enrollments before they are actually activated (approval step).
* See what courses a user used to be enrolled in for when they re-enroll in
  the same course, or a different run of that course.
* Have different "modes" of enrolling in a course, representing things like
  honor certificate enrollment, auditing (no certs), etc.

This change adds an is_active flag and mode (with default being "honor").
The commit is only as large as it is because many parts of the codebase were
manipulating enrollments by adding and removing CourseEnrollment objects
directly. It was necessary to create classmethods on CourseEnrollment to
encapsulate this functionality and then port everything over to using them.

The migration to add columns has been tested on a prod replica, and seems to be
fine for running on a live system with single digit millions of rows of
enrollments.

Hotfix/adam/help modal