Switch to external mock data repo

An idea I wanted to try out for a while: use a separate repo, installed via npm, for all the mock data: https://github.com/tidepool-org/blip-mock-data

Means we can have multiple mock data sets, even large ones, without fear of bloating the main Blip GitHub repo. `git clone` and `npm install` should stay fast for new developers.

What do you guys think?

Am using Jana's newest dataset from #91, so if we merge this we can close that one.