Refactor auth code to be cleaner and more extensible

We move all the auth handling, serialization and deserialization into a new AuthContext interface, and then standardize a registration model for handling of specific auth context types (user, robot, token, etc).

auth/credentials.py

@@ -7,15 +7,13 @@ import features
 from app import authentication
 from auth.oauth import validate_oauth_token
 from auth.validateresult import ValidateResult, AuthKind
+from auth.credential_consts import (ACCESS_TOKEN_USERNAME, OAUTH_TOKEN_USERNAME,
+                                    APP_SPECIFIC_TOKEN_USERNAME)
 from data import model
 from util.names import parse_robot_username
 
 logger = logging.getLogger(__name__)
 
-ACCESS_TOKEN_USERNAME = '$token'
-OAUTH_TOKEN_USERNAME = '$oauthtoken'
-APP_SPECIFIC_TOKEN_USERNAME = '$app'
-
 
 class CredentialKind(Enum):
   user = 'user'