Move out datengartens into their own section

- /datengarten/index.html returns a blog-like list
- /datengarten/index.ics returns a VCALENDAR
- /datengarten/index.xml returns a schedule.xml-style view

No more pretty rendering includes in the content pages,
some more cleanup in the front matter

content/datengarten/35.md

@@ -5,10 +5,12 @@ no: 35
 title: "Datengarten 35"
 subtitle: "Stalker - das Buch, der Film, das Game, der Mensch"
 date: 2009-11-05T00:00:00+02:00
-event_date: 2009-11-05T20:00:00+02:00
+event:
+  start: 2009-11-05T20:00:00+02:00
+  end:   2009-11-05T22:00:00+02:00
 location: CCCB
 speaker: Bernd, Andrej
-language: Deutsch
+language: de
 streaming: false
 ---