Fixes the double initialization of all action monitor classes
Removes a dead file
Moves all preview logic to happen on preview template redirect instead of on save_post to make previews more reliable
Adds a ?gatsby_preview=true param to the preview template url as WP core only treats every second preview load with no content changes as a preview but we want every preview load to be treated as a preview
Removes legacy preview as the maintenance burden will be too high to keep Content Sync & the old WP preview loader
This PR: