R1:d625f94c5553

Provide a markup protocol whitelist for Phabricator

Summary: See T548 and D996. Makes Phabricator configure the remarkup engine so
http:// and https:// get linked. Also make the "named link" syntax respect the
whitelist.

Test Plan:
- Whitelisted URIs (they get linked).
- Other URIs (not linked).
- Whitelisted, named URIs (linked).
- Other, named URIs (treated as phriction links).
- Actual phriction links (work correctly).

Reviewers: jungejason, nh, tuomaspelkonen, aran, benmathews

Reviewed By…