R1:4bfbd209b224

Fix XSS hole in YouTube remarkup rule

Summary:
The source wasn't properly escaped.

Test Plan:
Made a comment like "http://youtube.com/?v="></iframe><h1>!!!</h1>"

Reviewed By: mroch
Reviewers: tomo, mroch, tuomaspelkonen, aran, jungejason
CC: aran, mroch
Differential Revision: 516