revert !58 category paginating: it fails with stacktrace

I'll have to revert this, I'm out of ideas.  Its a strange issue.
Basically the `date_to_xmlschema` filter in __pages/feed.xml_ is
getting a list of `Page` instances instead of a `datetime`.

!58 (comment 361936012)

This reverts commits:
1e9a5a4c
fa7e69e7
2 jobs for master in 6 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #856944243
debian_buster

00:00:59

passed #856944241
ruby_latest

00:06:11