Implement faster resolver for global campaign diff stat

Created by: eseliger

Before we had to load all the changesets, which including metadata can take quite a while for larger campaigns (quite some data to be sent). This approach asks the DB directly, while respecting repo permissions, and is close to instant, removing the need to lazy-load the diff stat on the campaigns page.

TODO: I can't figure out why the test fails, if you have an idea, please comment :) Otherwise I'll debug that next. Opening up for review anyways, since I assume the test fix won't change much.