Replace gitlab-rails sources by heptapod-rails

This is enough to produce packages with Heptapod frontend assets properly compiled (thumbnails and other images with discriminating hashes in their names, minified JavaScript code etc).

This is harder than it looks, because omnibus-gitlab very much wants to fetch the sources with Git and its own upstream Omnibus doesn't have a HgFetcher.

An intermediate step might be to switch to a manually prefetched directory in /var/cache/omnibus.

Then perhaps, we could just reuse parts of install_heptapod.py to do that fetching for us (and move heptapod_revisions.json to the omnibus-heptapod project).