Return a user key/id from the user rest endpoint (BB-7739)

OAuth does not return a key or id for a particular user. After authenticating I'm getting back something like this for my own account..

{ "username": "lipis", "first_name": "Panayiotis", "last_name": "Lipiridis", "display_name": "Panayiotis Lipiridis", "is_team": False, "avatar": "https://secure.grav...", "resource_uri": "/1.0/users/lipis" }

But since you can change the username (I did that for example) it's
not good for my own application if that particular user going to user
Bitbucket login again. I need something more unique and unchangeable
from your side so I'll be able to associate that in my internal
database.