API: extend user object to link to trackers & tracking users

It would be great to get a list of ID's or at least an array of user pubkeys for all trackers and tracking users of a particular user.

The issue asks for the user object to expose tracker and tracking-user IDs or public keys, but names no API entry point, files, or tests. First establish how the user object is represented and exposed, then define whether IDs or public keys are required; done means a particular user's response includes the requested tracker relationships.

Written by the indexing model from the issue text.