Add video upload feature to Studio

This still needs to be audited for accessibility.

Please review: @nasthagiri @andy-armstrong @frrrances @talbs 
FYI: @clrux @doctoryes @karadhya 

*JIRA Tickets:*
* https://openedx.atlassian.net/browse/MA-28
* https://openedx.atlassian.net/browse/MA-85
* https://openedx.atlassian.net/browse/MA-86

*UX Design:* https://edx-wiki.atlassian.net/wiki/display/UX/Video+Uploads+in+Studio
*Functional Spec:* https://openedx.atlassian.net/wiki/display/MA/Video+Pipeline
*Sandbox:* http://studio.video-pipeline.m.sandbox.edx.org/

@clrux, given the extra context you gave me via HipChat, I'm fine with this staying as a %-based value. Is there a way to have the vertical padding also use a %-based value? Also, a comment inline with this noting why the % is needed would be good for future folks working around here.