Changes:
  - make the XML closer to spec (RSS does not pass w3c's validator, but works)
  - fix dates (RFC3339 for Atom, doc says RFC822 for RSS but RFC1123 is closer)
  - fix attachment/enclosure links (but see below)
  - set feed item title to post's "summary" if present
  - pruned several elements that validators did not like
    - examples: ap_enabled, user banner urls.

Specs:
  - https://www.rssboard.org/rss-specification
  - https://validator.w3.org/feed/docs/atom.html
  - https://www.intertwingly.net/wiki/pie/Rss20AndAtom10Compared

Validators:
  - https://validator.w3.org/feed/
  - https://rssatom.com/feedvalidator.php

Attachment/enclosure links should have a "length" field (mandatory
according to the spec).  This is not present in the object's data
map.