Studio: cleaning up/removing unused sass

* removing unused font-family-centric Sass variables
* removing unused graphic images and Sass references
* clean up of style removal fallout
deleting more things
Remove extra large-FOO-icon.png files
Removed more files from cms
Deleted some unused images under common
content-types is no longer around
re-add search-icon.png
allow for a customized region on the receipt page that is microsite brand specific
Unit Test Improvements
TE-489
add missing file
rename file
Remove "ADMIN_MEDIA_PREFIX" (deprecated in Django 1.4)

Delete lms/static/admin resources as they are no longer needed
for the admin page as of Django 1.4

Moved two files that were referenced elsewhere:
lms/static/admin/js/jquery.min.js -> lms/static/js/admin/jquery.min.js
lms/static/admin/js/urlify.js -> lms/static/js/admin/urlify.js
Remove unused static assets
Remove old 404 page
No need to use which on a .py file
Merge pull request #4806 from edx/talbs/studio-stylesweep-unused

Studio: Removing older/deprecated style rules
Implement shim for mobile video events.

This PR addresses the following issues:

1) All requests return a 200 OK unless there is an authorization failure. This is deliberate in case the secret key is compromised.
2) Push all of the nasty logic necessary to generate compatible video events into the LMS instead of trying to do that mapping on the mobile devices.
3) Stop using the deprecated "action" field in the segment.io event. According to their support team this field should not be used anymore and is just around for backwards compatibility reasons.

Fixes: AN-3818
Abstract asset methods into own interface class
then impl the read ones as NotImplementedError placeholders on xml modulestore
PLAT-70
fixup! Remove "ADMIN_MEDIA_PREFIX" (deprecated in Django 1.4)
Remove documentation files moved to edx-documentation repo