Support GitLab in campaigns

Created by: mrnugget

Steps:

• Update src-cli to extend this check
• Extend SupportedExternalServices
• Implement the ChangesetSource interface on repos.GitlabSource (see repos.GitHubSource and repos.BitbucketServerSource as examples)
• Add gitlab support to the workers.go so that changesets can be created by us.
• Handle gitlab.MergeRequests in the switch statements in campaigns/types.go

At this point, we should be able to create changesets and display them (but their External* fields should be empty).

Next step is to support ChangesetEvents:

• Load the events in the ChangesetSource
• Support gitlab in the ChangesetEvents
• Handle gitlab metadata in the state computation switch statements
• Handle gitlab events in the computeHistory function (which is used for the burndown chart and the state computation above)
• Last: add webhook support