Basic Mobile API (v0.5) and Video Abstraction Layer integration.

Note that the features in this release are opt-in, and course and video
behavior will remain the same unless a course explicitly opts in.

Major pieces of functionality with this commit:

Allows the listing of a user's enrollments, course videos, and updates. In
order to make a course available for mobile use, course staff must explicitly
set the Course Advanced Setting "Mobile Course Available" to true. Course staff
will always see their own courses through the Mobile API regardless of this
setting, but students will only be allowed to see a course through the Mobile
API if this setting is set to "true". By default, a Course will *not* be
available for mobile use.

This is a Django app for video resource management. It is completely optional,
and is intended to allow video and operations teams to create new encodings of
videos (e.g. low res for mobile) and change CDNs without having to edit course
data directly. Course teams can now use a "EdX Video ID" setting for Videos,
which will leverage VAL. Video units that do not fill in an "EdX Video ID" will
behave exactly as they always have.

* The Mobile API is enabled with the ENABLE_MOBILE_REST_API feature flag.
* VAL is enabled with the ENABLE_VIDEO_ABSTRACTION_LAYER_API feature flag.
* VAL and the Mobile API both require ENABLE_OAUTH2_PROVIDER).
* The Mobile API is a read-only API, but VAL requires database migrations.
* Applications that make use of either the Mobile API or VAL must be registered
  with the OAuth2 provider app in Django Admin.
Merge pull request #5297 from edx/ormsbee/mobile_release

Mobile API release