Only automatically link protocols from a protocol whitelist

Summary: See T548. Whitelist permitted protocols to close a hole with
javascript:// links.

Test Plan: Wrote a comment with links in it, some using permitted protocols and
some using other protocols. Only the permitted protocols were linked.

Reviewers: jungejason, nh, tuomaspelkonen, aran, benmathews

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 996