Content Libraries: Fix Help links in Import/Export templates

Addresses problem with 'up' and 'down' errors within SOL-289

DOC: ContentLib Help links for user access

Bypassing E-Commerce API if user is enrolled in course

If a user is already enrolled in a course, the API is bypassed and a 409 status is returned.

Make sure slashes in JSON content don't end script tags. PLAT-462

The string "</script>" in JSON data would end the script element we're
embedding the data in.  To make sure the data doesn't disrupt the
script, we escape the slash as \/ .

Merge release to master for 20150317 release.

Reverting the change to add a 201_CREATED status to the enrollment API.

Fix or remove tearDown methods that don't use super.

MA-7: A/B test and cohorted content support

UX-1642 Making visual tweaks and enhancements to the logistration form

Update newrelic agent for monitoring celery tasks

XCOM-45 Enrollment API supports creating modes other than honor.

Copy changes to the activation email

Downgrade CORS middleware warning to info

Certificates: Revising Web Templates + UI for Open edX Use

Fixing tests, cleaning up POST handler to be easier to read.

Wording fixed for unit test comments.

Validating event emission by the commerce app

Fixed CourseMode Filtering Bug

Added SKU column to CourseMode table in Django admin

Allow services to ignore user throttling. Set a more realistic throttle for users.

Commerce code has now been updated to exclude CourseMode objects with SKU set to empty string (in addition to null).

Remove useless override_settings of MODULESTORE

Add show/hide previews button to Content Libraries