Add support for Kiln and FogBugz as hosting services.

Kiln and FogBugz are services provided by Fog Creek for repository
hosting and bug tracking, respectively. Kiln does not support any
built-in endpoints for Mercurial file fetching, and the Git hosting is
not compatible with the raw file URL support.

This change adds support for working with repositories stored on Kiln,
and for easily setting up bug tracking at FogBugz.

The Kiln API has almost everything needed for fetching commits for the
New Review Request page, but is just useless enough where we cannot
provide that feature. The primary reason being that the API only allows
paginating from the oldest commit to the newest, and not vice-versa. It
also doesn't really allow us to start at any particular revision. So for
now, we just have basic support.

This also does not support Kiln repository aliases.

Reviewed at https://reviews.reviewboard.org/r/6605/