identify users with usernames

[speezepearson]

Currently, users are uniquely identified by UserIds:

### message UserId {
  oneof kind {
    string username = 1;
    // string oauth_google_acct = 2;
    // string oauth_facebook_acct = 3;
  }
}

This produces a couple of complications:

• Knowing somebody's UserId can tell you things like their email address, or how to find them on social media. They probably don't want that. But then, when how do we display users? E.g. when somebody hits GetPrediction, how do we tell them who the creator is?
• oneof actually means "zero-or-one-of", so there need to be conditional checks in lots of places.

So, instead, let's say that every user has a unique username.