404, Backend overview, Clear

* 404 page no longer creates a new 404 if the site contains 0 or > 1
settings (less garbage)
* Backend overview doesn't crash if the site contains 0 or > 1 settings
* Clear: patch to clear entire site now clears all know tables for the
environment (comments, commentmeta, links, terms, term_relationships,
term_taxonomy, postmeta, posts)