Ensure API.query URIs can traverse one-to-many Relationships

[enobrev]

Currently, to get all the themes for all the currently selected cities, the query looks something like this:

API.query([
    '/cities',
    '/themes/{cities.theme_id}'
]), () => {}};

This is fairly reasonable if you know the relationships, but not ideal or consistent, especially since many-to-many relationships can be figured out just fine.

Ideally, this could also be done thusly:

API.query([
    '/cities',
    '/cities/{cities.id}/themes'
]), () => {}};

The resulting query should be the same.