Fix ambiguous URI parsing in Youtube Remarkup rule

Summary:
Fixes T12867. Also:

• Simplify the code a little.
• Stop mutating this on text/mobile -- there's no inherent value in the "youtu.be" link so I think this just changes the text the user wrote unnecessarily.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12867

Differential Revision: https://secure.phabricator.com/D18149