Don't allow authors' <style> elements

Previously, the styles from an entry with CSS could leak out into the
rest of the front page.  Better would be to munge the CSS rules to
prepend a per-entry "#entry-NNNN" to each selector, but that's quite
tricky; this seems like an improvement for now.
Delete empty <p> elements

This is particularly useful given the previous commit: deleting
a <style> element often leaves an empty <p> behind.  But we have
some entries which contain an empty <p> anyway; this change fixes
those.