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=...}.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D19118