Allow response to vexatious requests to be submitted without response input

Silently replaces the blank public_part field with the text
"Rejected as vexatious" to preserve the expected data structure.
Keeps the submitted response if the responder has provided one.
Remove delete option from admin forms
Route "Respond" link to request_path as per the Request title link

Also removes new_response_request_path route as it's no longer used
Change button text to "Submit Response"
Prevent response being submitted with request state of New