Move the main webapp repo over to git (though not its branches!)

Summary:
I'd like to figure out how to handle Website/Group in a principled
way, but I don't really understand what's going on there yet, so I
just hard-code that we're downloading Website/Group/webapp as a git
repo, and leave the others for later.

Test Plan:
Ran
   ./update_phabricator_repositories.py -n /var/tmp/
and saw it say:
   Adding 1 new repositories to phabricator
   Adding new repository Website/Group/webapp: url=ssh://khanacademy.kilnhg.com/Website/Group/webapp, callsign=GW, vcs=git, destdir=/var/tmp/git/Website/Group/webapp
and also
   Repository https://khanacademy.kilnhg.com/Code/Website/Group/webapp has been deleted: run (on toby):
   env PHABRICATOR_ENV=khan ~/internal-webserver/phabricator/bin/repository delete WE

Reviewers: chris, alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D4160