Backdrop sends Bearer token to Stagecraft

TODO:
- [x] add tests for `_get_json_url`
- [x] fix the fact the above function returns `None` ever


The `/data-sets` endpoint is protected by a shared bearer token.
Backdrop must send it when it requests data set metadata.

Quite a lot of test refactoring.

Related to https://www.pivotaltracker.com/story/show/67103208
[#67103208]