Accept and route VCS HTTP requests

Summary:
Mostly ripped from D7391, with some changes:

• Serve repositories at /diffusion/X/, with no special /git/ or /serve/ URI component.
  • This requires a little bit of magic, but I got the magic working for Git, Mercurial and SVN, and it seems reasonable.
  • I think having one URI for everything will make it easier for users to understand.
  • One downside is that git will clone into X by default, but I think that's not a big deal, and we can work around that in the future easily enough.
• Accept HTTP requests for Git, SVN and Mercurial repositories.
• Auth logic is a little different in order to be more consistent with how other things work.
• Instead of AphrontBasicAuthResponse, added "VCSResponse". Mercurial can print strings we send it on the CLI if we're careful, so support that. I did a fair amount of digging and didn't have any luck with git or svn.
• Commands we don't know about are assumed to require "Push" capability by default.

No actual VCS data going over the wire yet.

Test Plan:
Ran a bunch of stuff like this:

$ hg clone http://local.aphront.com:8080/diffusion/P/
abort: HTTP Error 403: This repository is not available over HTTP.

...and got pretty reasonable-seeming errors in all cases. All this can do is produce errors for now.

Reviewers: hach-que, btrahan

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7417