Support GitLab self-hosted

When we launched Quests we only had support for Gitlab.com, on the theory that self-hosted GitLab would require more user support and we weren't sure what the business model for Quests would turn out to be. This has been a fairly common request, but not something we use ourselves so it hasn't been added as of yet. Now that it's open source, this is a pull request we'd readily accept. 😊