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: jungejason

CC: aran, jungejason

Differential Revision: 997