R1:05a4c55c52be

Explicitly add rel="noreferrer" to all external links

Summary: See D19117. Instead of automatically figuring this out inside `phutil_tag()`, explicitly add rel="noreferrer" at the application level to all external links.

Test Plan:
- Grepped for `_blank`, `isValidRemoteURIForLink`, checked all callsites for user-controlled data.
- Created a link menu item, verified noreferrer in markup.
- Created a link custom field, verified no referrer in markup.
- Verified noreferrer for `{nav href=...}`.