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

Author: epriestley

src/applications/files/config/PhabricatorFilesConfigOptions.php

@@ -155,6 +155,7 @@ public function getOptions() {
             'List of MIME types which can be rendered with an `%s` tag.',
             '<audio />')),
       $this->newOption('files.video-mime-types', 'set', $video_default)
+        ->setLocked(true)
         ->setSummary(pht('Configure which MIME types are video.'))
         ->setDescription(
           pht(