Fixes #73 loading user by username via json service

I finally got a version of the app going, and these changes fixed the problem.

The issue was the assumption that you could load a user profile with /users/<username>, which has never really been true, but in recent years we haven't been using it at all. But you *can* use the user load service with a username instead of a uid, so I changed it to do that. 

Unfortunately, I had to do this work in Android Studio, which imported the entire thing with a different build system and everything else, so I'm unable to test this as is. I've tested the changes, but not as committed here.