Direct GitLab integration v1

Created by: sqs

Draft announcement: https://github.com/sourcegraph/about/pull/41

Milestones:

• Spec out how we are going to include JS in GitLab @ijsnow
• Write PR description that contains roadmap @ijsnow
• mocks @francisschmaltz
• Open the PR when the description and mocks are ready @ijsnow
• Add db table to store Sourcegraph URL and access token @ijsnow
• Implement UI for storing token and url @ijsnow
• Implement CodeHost interface in GitLab @ijsnow
• Script that runs extensions using acces token and CodeHost from GitLab @ijsnow