"Don't use default CSS" StorySetting

* Added a StorySetting that lets you remove the aforementioned stylistic
CSS for each story format, giving CSS authors a clean(er) slate to work
with. (Note: Responsive currently doesn't implement this.)
* Engine: Added an extra few checks to main()'s sanity checker, to
ensure that macros, prerender and postrender aren't replaced with
non-objects, or (for the latter) given non-function properties.
* The Help menu for the Passage editor now contains different items if
you're editing a script, stylesheet or regular passage. (Note: this is
only set when you open the pane - changing the passage's role by
altering its tags won't alter the menu immediately.)