Issue #3623899: Make forms a described and generated part of the model, submission included

A submission endpoint that declares nothing is described by the contact form of the site: the form display of the "api" form mode when the site keeps one, the fields the site added otherwise. The same description is what it accepts, what it stores and what the mail lines say. The planner gives every contact form a submission endpoint and no longer plans detail or listing endpoints for contact messages. A reference to a contact form is answered with the description of that form. Generated submission endpoints declare their path as "create", which is where REST serves a POST, and a page address is stored into a link field as an internal URI.

Closes #3623899