Add bokchoy test and changes in html

Corrects duplicate main element wrapper on Wiki article template

Updates and corrects href links to readthedocs

Merge release to master for 2016-05-17 release

AC-405 calculator image to icon

AC-399 adding alt for circuit schematic controls

AC-396 updating courseware nav to display correctly on high contrast displays

User administration updates

OSPR-1155

Include course price in course_about template context if either the ecommerce service is enabled or shoppingcart is enabled

Fix flaky bok-choy test

Safe template cleanup in settings.html

Use CourseOverview instead of modulestore.

asadiqbal08/WL-420 Updating drag and drop version. 
merging. 

Display correct timezone on timestamp for question submission

Ahsan/removal underscore string from commerce js

Fixes for courseware date formatting/translation.

ECOM-4281

Add skip_clean option to pa11ycrawler

refine from_addr length limits (TNL-4264)

Updated URLs for help topic links that had become stale. Per DOC-2568.

The view for submission history used to grab the timezone from the database (UTC) and
appeded the TIME_ZONE variable after (could be any time zone). Adjusted displayed
timedate to the TIME_ZONE variable. Added unittest checks the localized datetime
object used by submission history for the expected hour difference.

Add referral tracking for new registrations.

Move ENABLE_INSTRUCTOR_EMAIL to admin panel

ECOM-4325

Moves ENABLE_INSTRUCTOR_EMAIL and REQUIRE_COURSE_EMAIL_AUTH from settings files
to admin-accessible configuration models. This allows for the bulk email settings
to be modified without a new AMI deploy. See TNL-4504.

Also updates tests:
    -python tests mock out the new configurations in place of the old settings
    -lettuce test has been moved to bokchoy
        (note that there was some loss of coverage here - the lettuce tests had
        been doing some voodoo to allow for cross-process inspection of emails
        messages being "sent" by the server, from the client! In discussion with
        testeng, this seems outside the realm of a visual acceptance test. So,
        the bokchoy test simply confirm the successful queueing of the message,
        and leaves the validation of sending messages to the relevant unit tests.)
    -bok choy fixture has been added, to replace the settings in acceptance.py
    -lettuce and bok choy databases have been updated to reflect the backend changes

The new default is to have bulk_email disabled, we'll need to call this out in the
next OpenEdx release to ensure administrators enable this feature if needed.