Extending feature for schema validation

• Task 1 Improve the message raised by schema validation architecture when the handler class contains schema and the name of the handler class is also present in the list HANDLER_CLASS_NAMES_WHICH_STILL_NEED_SCHEMAS. Explanation: Currently the error raised when the above condition staisfies is something – “Nonetype object has no attibute …” After Modification: Proper error message should be raised which is self explanatory that handler class name(s) must be removed from the list.

• Task 2 Allowing head request for Schema validation. Explanation: Currently Oppia supports GET/PUT/POST/DELETE request methods but sometimes in production HEAD request is also initiated. Thus allowing schema validation architecture to skip validtaion when it encounters request from HEAD method.

• Task 3 Introduce lint checks to enforce developers to use self.normalized_payload and self.normalized_request instead of self.payload and self.request while writing new handlers / schema.

Reference to some useful links:

1. How to write schema validation for handler arguments

[vojtechjelinek]

This was closed in #15044. Thanks @aasiffaizal

[vojtechjelinek]

Yeah, it is not yet deployed, but I marked that PR to be cherry-picked into the relase.