Implemented process_response method to clear requestcontext global variable in mako middleware.

LMS-6622

Implemented process_response to clear global "requestcontext" variable in mako middleware and added unit test.
yes, although I'd probably rename `thread_local` to `REQUEST_CONTEXT` and `request_context` to `context`, or something like that, just to give a better indication of what that thread local is going to be storing.