batches: updating a draft MR on GitLab results in the MR no longer being a draft

Created by: LawnGnome

The update action for GitLab changesets doesn't check whether the changeset is intended to be a draft or not before setting the title on the MR, which means that changesets can be accidentally undrafted because the raw changeset title doesn't include the WIP prefix required for GitLab to mark a changeset as a draft.

The fix here is to have the reconciler figure out if the changeset is a draft, then inform the changeset source accordingly.