Add API endpoint returning consumer information (bug 964524)

https://bugzilla.mozilla.org/show_bug.cgi?id=964524

This new API endppoint returns the region slug, and if the user is logged in, the list of apps ids he has installed, developed and purchased.

Note that we can't remove the user-specific info from the endpoints that use them for the moment, because some API users might be using it, but that's a topic for another bug.

r? @cvan