Support size=thumb parameter for embedded video files in Remarkup
Summary:
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
Test Plan: Upload a video file; put `{F1,size=thumb}` and `{F1}` into a task comment; resize viewport width.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16042
Differential Revision: https://we.phorge.it/D26080