Lock files.video-mime-types config option for consistency

Summary:
This is a consistency change to make this option consistent with audio-mime-types, image-mime-types and icon-mime-types, all of which are locked.

(They're locked because SVG is definitely dangerous, and other types might be dangerous or might become dangerous in the future, although I'm not aware of any actual dangers from video types today.)

Test Plan: Viewed files.video-mime-types in Config, saw it was locked.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17454