Sanitize: figures and anchors
Add missing <p> tags

This is to silence the validator erro "text not allowed here".

<blockquote> closes <p>, and EPUB requires some element inside
<blockquote>, so just shift them around. The previous code resulted
in empty <p> elements using a modern parser.
Sanitize: fine-tune the figure heuristic
Clean up the Apollo 8 invitation
Remove superfluous and invalid <center>
Sanitize: allow the <a name=""> rewrite for <h3> as well
Move note 45 from chapter 13-5 to 13-4, where it is referenced
Add a validate make target using epubcheck 3.0
Add all the images to EPUB
Remove the EPUB before ziping it, just to be safe
Fix some incorrect URLs

These are the last errors reported by epubcheck,
the EPUB file now passes validation.