Add a CMA::Filename

* Capable of making a place for things to be saved to a `CaseStore`
  with `Filename.for`
Allow CaseStore to make its own URLs

* Where a filename is not supplied, use Filename.for(case.original_url)
Filename should append .json
Allow a CaseList::Page to save its cases to CaseStore
Update CC::Crawler so it navigates everything

* Doesn't save everything yet
* Does save case titles/original URLs in the right place