Migrate the datahub build pipeline to GitLab

Previously these blubber files were maintained within the analytics/datahub repository in gerrit, within a wmf branch.

Now we have un-forked our build pipeline from the upstream project, so each of the jobs downloads a pristine copy of the source and builds from that.

This migration to GitLab represents the final stage in the un-forking, since we include only the blubber configuration files and a .gitlab-ci.yml file defining the pipeline.

Bug: T341194