Make it easy to access `first_contribution` for merge request resources
This would be using gitlab-org/gitlab#24742 (closed)
For now the data can't be accessed via the list of merge requests API, but we can add it to the individual merge request resource, which will be one API request for each merge request, which will be slow, but that's ok and we're doing that a lot already, proven acceptable.
/cc @rpaik