Allow Phabricator to serve Mercurial repositories over HTTP

Summary: Ref T2230. This is easily the worst thing I've had to write in a while. I'll leave some notes inline.

Test Plan: Ran `hg clone http://...` on a hosted repo. Ran `hg push` on the same. Changed sync'd both ways.

Reviewers: asherkin, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7520