Query changeset stats from database

Created by: eseliger

This requires us to load all changesets, which we won't do anymore once we can fetch from the database using DB based authz. Hence, I propose we move away from iterating in the application layer and instead asking the DB for it. Also, on my local instance where I have 10k changesets in one of my campaigns, it significantly slows down page loads, worth mentioning even on the global campaigns list page, because we display some of the stats there. This get's it down to some ms again.