SEO: Integrate feature specified in PL in page

This adds features passed with the PL to the indexed page. It will integrate the html popups of the passed feature(s) as part of the snapshot paste.

Example PL: map.geo.admin.ch/?ch.bafu.schutzgebiete-paerke_nationaler_bedeutung=7,12&ch.swisstopo.fixpunkte-agnes=HUTT

This likely conflicts with #1369. Whichever is later needs a rebase.