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
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
Merge pull request #5781 from mulby/mobile-eventing-v2

Implement shim for mobile video events.