details page: Hide fields that don't make sense for sources

This makes installing a .flatpakrepo file much nicer as we no longer
show a bunch of fields with "Unknown" in it.
1 job for <span class="ref-name">wip/kalev/flatpakrepo-hide-unknown-fields</span> in 23 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #232253
build-gnome-software

00:23:21