User configurable reports

the plan is to pilot/QA on this on prod so I'm putting it out for code review now. I'm guessing it's easiest to review as a whole, though the commits should represent ~atomic actions.

I think it's good to merge whenever code review passes.
i think the reason i was wondering this was in the scenario if you ever wanted to have multiple `DatespanFilter` instances on the same page and needed to override the parameter slug in order for that to work properly.