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.

remove unused code from xml_import.py

Changelog updates

Fix/adam/i18n templates

remove 'preview' from urls in enrollment emails sent while previewing a ...

Change course create form to asynchronous validation.

Change locators to a restful interface.

Fix error when there is an invalid YouTube property in a video module

Make descriptorsystem inherit from Runtime

Conflicts:
	cms/djangoapps/contentstore/views/preview.py

XBlock integration for Studio: student view

cleanup

instructor dash fixes

Coffee compile command now executes for just *.coffee items.

Add new setting to cms project

Migration videoalpha module to one main video module

Change console logging stream from stdout to stderr

Enable beta dashboard, add copy, add i18n

Calc module changes: previewing and <formulaequationinput>

To see individual commits, see tag peterb/formula-preview/presquash