Only show site preview form on request so {cache_form} doesn't get too big [#2193009]

Updated: Comment #N

Problem/Motivation

Currently the SPS Condition Preview Form gets rendered and cached on every page load. This quickly grows the {cache_form} table on a site with many editors hitting the site. Even though their is a 6 hour expiration on the entries in {cache_form} it would be beneficial to not have this rendered uneccesarily.

Proposed resolution

Put a "Preview Site" link in the admin menu and load the form via an Ajax request, then allow the user to toggle it open and closed. Once in preview mode, then render on every page load until site preview is disabled.