Notify humans about conflicts

**WIP - do not merge**

Retcon needs to notify humans when it finds a conflict it can't handle automatically (i.e. all of them). This branch will develop this capability.
It's relabelling the diffs with (). The Diff type takes a completely arbitrary parameter and we're about to serialise it and stuff it in a database. I couldn't be bothered thinking about the "correct" solution (maybe add ToJSON and FromJSON constraints on the param) so we just did this.

I'll add comments to make clear what's going on here and that it needs changing at some point.