Embed videos in Remarkup at actual size

Summary:
Ref T6916. Current rules tend to make videos gigantic. Just embed them at actual size, scaling them down if they're too big to fit.

Browsers generally provide some kind of "expand / fullscreen" element automatically anyway.

Test Plan: Viewed videos locally, saw them sized a little more reasonably.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6916

Differential Revision: https://secure.phabricator.com/D16076