campaigns: allow changeset diffs to be downloaded
Created by: LawnGnome
I realised after this morning's planning meeting that I knew how I wanted to do this, so even though it was planned for the next sprint, I figured I'd rather get it done while it was fresh in my mind.
This is pretty rough in terms of styling (ping @rrhyne), but it does work. Behold!
Closes #16098 (closed).