Add Videos to Remarkup

Summary: Ref T6916. Added video to remarkup using D7156 as reference.

Test Plan:

• Viewed video files (MP4, Ogg) in Safari, Chrome, Firefox (some don't work, e.g., OGG in Safari, but nothing we can really do about that).
• Used alt.
• Used autoplay.
• Used loop.
• Used media=audio.
• Viewed file detail page.

Reviewers: nateguchi2, chad, #blessed_reviewers

Reviewed By: chad, #blessed_reviewers

Subscribers: asherkin, ivo, joshuaspence, Korvin, epriestley

Tags: #remarkup

Maniphest Tasks: T6916

Differential Revision: https://secure.phabricator.com/D11297