R1:411cf1345763

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

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