Migrate to the GitLab GraphQL API once the minimum version is 12.0

Created by: LawnGnome

#11757 greatly expanded the calls we make to the GitLab API, particularly around merge requests. Those calls could be more efficiently made using the GitLab GraphQL API: currently, we need to make three calls per MR to get the MR, notes, and pipelines; with GraphQL we could potentially make one call for an entire campaign's worth of MRs (subject to GitLab's own rate limiting), and get better label information at the same time. (The REST API does not provide a way to get label descriptions or colours from a merge request.)

The GraphQL API only became available without enabling a feature flag in GitLab 12.0, so that would be the minimum version we would have to support before this becomes feasible.