Add logic to disable Jetpack contact form on jetpack.wp-a2z.org

[bobbingwide]

The sample contact forms required to produce examples of Jetpacks form fields should work but do nothing .

Requirement

• In jetpack.wp-a2z.org we want the form to basically do nothing on submission.

Proposed solution

• Find the right filter that will prevent the form from being submitted
• It would be nice if we don't even get entries created in Feedback.

Notes: Returning true to jetpack_contact_form_is_spam doesn't work as a short circuit.
How about returning a wp_error!

[bobbingwide]

Returning a WP_Error seems to work as follows:

• The Feedback post is not created
• No email is sent
• The page is re-displayed.
• The WP_Error is forgotten about.
  The page is simply re-displayed.