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/test/test_oauth.py

@@ -21,7 +21,7 @@ def test_valid_oauth(app):
   token = list(model.oauth.list_access_tokens_for_user(user))[0]
 
   result = validate_bearer_auth('bearer ' + token.access_token)
-  assert result.oauthtoken == token
+  assert result.context.oauthtoken == token
   assert result.authed_user == user
   assert result.auth_valid
 
@@ -32,7 +32,7 @@ def test_disabled_user_oauth(app):
                                                       access_token='foo')
 
   result = validate_bearer_auth('bearer ' + token.access_token)
-  assert result.oauthtoken is None
+  assert result.context.oauthtoken is None
   assert result.authed_user is None
   assert not result.auth_valid
   assert result.error_message == 'Granter of the oauth access token is disabled'
@@ -44,7 +44,7 @@ def test_expired_token(app):
                                                       access_token='bar', expires_in=-1000)
 
   result = validate_bearer_auth('bearer ' + token.access_token)
-  assert result.oauthtoken is None
+  assert result.context.oauthtoken is None
   assert result.authed_user is None
   assert not result.auth_valid
   assert result.error_message == 'OAuth access token has expired'