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/decorators.py

@@ -69,7 +69,7 @@ def require_session_login(func):
   @wraps(func)
   def wrapper(*args, **kwargs):
     result = validate_session_cookie()
-    if result.has_user:
+    if result.has_nonrobot_user:
       result.apply_to_context()
       metric_queue.authentication_count.Inc(labelvalues=[result.kind, True])
       return func(*args, **kwargs)