Build feed contents using templates

This fixes the issue where imported posts had no contents in the feeds.

There's some annoying repetition in here obviously, but we could fix
this by adjusting our fork of lektor-atom to take a `[default]` config
section into account. Currently the defaults are hard-coded in the
plugin.
2 jobs for main in 6 minutes and 21 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Test
passed pages #44656

00:06:21

 
  Deploy
passed pages:deploy

00:00:00