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

Valera/bugfix tabbing video controls

Now you can tab through all of the controls in Video forwards, and then tab backwards.

An old TODO item was done. It turns out a simple case of calling
the method to bind handlers after the Spees dialog was re-rendered.

Cleanup some xmodule tests imports and data directory location

added video changes to the changelog

Add mgmt cmd to generate anonymized ID mapping

Fix/adam/lms help button

Enable instructor dashboard in lms/envs/dev

Fixups to Nate's anonymized id mapper, repairing pep8 and pylint errors,
and rebasing on recent master.

So that instructors have easy access to the mapping from anonymized
IDs (a simple MD5 hash of the user ID that's used when integrating
with third-party services like Qualtrics) to user IDs, we have a
simple Django management command to generate a CSV mapping. To run,
use the following:

rake django-admin[anonymized_id_mapping,<system>,<env>,<course_id>]

And, of course, substitute the appropriate values for <system>, <env>,
and <course_id>. (E.g., lms, dev, and MITx/6.002x/Circuits)

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.