feat: gitlab to gitlab mirroring

[cmelone]

I was able to map most of the features we've written for GitHub->GitLab mirroring.

A couple limitations to discuss:

• we cannot post statuses to forks
• how to register repositories with gitlab as a source (there's no equivalent to a GH app)
• gitlab makes it difficult to connect comments to commit ids, current way is through a "diff note" but we might consider implementing /hubcast approve <sha> to make it easier

I refrained from writing tests because I want to get some feedback on the architectural components, so the unit tests won't pass at the moment.

closes #115