When embedding a video in a task description via Remarkup's {F1} it takes up to 95% of the width which is often unwanted.
Should support the same {F1,size=thumb} parameter which embedded image files already support.
In case of image files the resulting width for TRANSFORM_PREVIEW is hardcoded to 220px in PhabricatorFileThumbnailTransform.php::generateTransforms().
That feels subjectively small for video thumbs so arbitrarily bikeshed to hardcoded 400px instead.
Closes T16042