m.htmlsanity: better consistency for collapsed <p>s in additional fields.

m.htmlsanity: better consistency for collapsed <p>s in additional fields.

Previously (and that's also how stock Pelican behaves) article summaries
and other FORMATTED_FIELDS had <p> collapsing disabled completely. That
made some inconsistencies in the HTML markup (i.e., a m.css note in
article summary would look slightly differently compared to a note in
article content because of the additional <p>).

Now <p>s are collapsed for FORMATTED_FIELDS as well and the HTML writer
has an additional exception that it *does not* remove <p>s from nodes
that are directly inside field bodies.