Factor out common Store code.

Also restrict the Store#request interface to require zero arguments.
Parameterized loads are interesting but cause too much of a bookkeeping
headache with memoization, and parameterization isn't necessary now (and
likely won't ever be).
GET /api/v1/study_locations.

This could use a feature, too.
Bring Stores.reload up to date.
Generate a study location manifest for dev and test.