Update to Jersey 2.x

https://www.pivotaltracker.com/story/show/52200823
This thing confuses me. There's something screwy with the way Jersey 2 picks between methods that share the same verb when a request without a Content-Type is received. In the last attempt, we just commented out the method. I suggest we should kill it entirely. I *think* it's only here to support a javascript-less admin UI.

@barmintor, @ajs6f ?