Put rel="noreferrer" on all nonlocal links

Summary:
Ref T4342. By default, insert `rel="noreferrer"` for links. We do not insert this if:

  - The caller provided an explicit "rel" attribute.
  - We recognize the link as an anchor ("#comment-2"), or as local to the current domain ("/path/to/resource").

Otherwise, add it in all cases.

Test Plan: Added and executed unit tests. (The javascript stuff which was touched a little bit has a pile of tests already, too.)

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4342

Differential Revision: https://secure.phabricator.com/D8711