Merge release back into master (hotfix-2015-04-03)

Hotfix: Add proxy to allow IE9 to make xdomain requests

Move debug toolbar to end of devstack middleware

Update SKU field help text and verbose name

The django debug toolbar circumvents other middleware with process_view
functions, so it needs to go at the end of our MIDDLEWARE_CLASSES
setting.

https://django-debug-toolbar.readthedocs.org/en/1.3/tips.html#middleware-isn-t-working-correctly

Fix duplication of Randomized Content Blocks

Update XBlock Hash

PLAT-520 Fix Asset store import of null values.

Adds an /xdomain_proxy.html endpoint that serves
the proxy file from the xdomain library.  This
allows IE9 users to iframe in the proxy page
to simulate a cross-domain request with cookies.

Reverts removed styling rule which broke certificate status message on the course dashboard

Add social sharing feature (Twitter, Facebook) to student dashboard

Ignore pylint error 7610 (translation-of-non-string) on where it is intended

Fixed invalid `usage key` errors which previously throw 500.

As some i18n features need ```ugettext``` or its shorcut ```_``` to be called on a non-string-literal, so the pylint check of ```translation-of-non-string``` should be explicitly disabled in such situations.

MA-410 Update video_module import to use target course_id.

Release

Edits to Advanced Settings UI text for Discussion Blackout Dates, Discussion Topic Mapping

Fix trailing whitespace issues

Fixed static pages removed on import for split course.

Decorate flaky test SOL-618

Platform APIs Doc

to address DOC-1434, DOC-1828