Adds stats object to foia hub model. refs #184

- Add stats obj to foia hub to store stats.
- Since, office-slug #197 was on its way to being merged, this is being submitted with that already accounted for. Migrations in this branch build off of migrations in that branch. 

(Someone let me know which way they want this pull request submitted. I will update appropriately.)


Why is this removing the `agency` field from `FOIARequest`? This field is used -- requests can either be on agencies or on offices. It looks unrelated to the rest of the PR, as far as I can tell.