ATOM feed: Make entry ID persistent across renames

Make sure that https://framagit.org/linuxphoneapps/linuxphoneapps.frama.io/-/blob/main/themes/lpadoks/templates/atom.xml puts the first URL in aliases into ID - else remains page.permalink as currently.

This should fix automatic feed based reposting on app id changes and - also - is the correct technical behaviour according to ATOM spec.