fix atom and rss feeds for users and tags

issue: #2980

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.