Add git hosting

As potential solution for #43 this adds [cgit](http://git.zx2c4.com/cgit/about/) for a web interface and [gitolite](https://github.com/sitaramc/gitolite) for hosting and access control.

Was unsure how to set them up for a common use case, so pretty much stuck with defaults. They both offer a wide range of settings though. Maybe someone with more experience hosting them could chime in with settings to use.

You'll have to run `cp ~/.ssh/id_rsa.pub roles/git/files/gitolite.pub` or similar to get working. Also need to add some documentation in general.

cgit exists at https://git.sovereign.local, and the gitolite settings are in the `git@sovereign.local:gitolite-admin` repo.