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/39.md

@@ -5,13 +5,14 @@ title: "Datengarten 39"
 no: 39
 subtitle: "Cubesat Projekt GLORIA"
 date: 2012-03-01T00:00:00+02:00
-event_date: 2012-03-01T00:00:00+02:00
+event:
+  start: 2012-03-01T00:00:00+02:00
+  end:   2012-03-01T00:00:00+02:00
 location: CCCB
 speaker: Sean/Raphael
-language: Englisch
+language: en
 streaming: false
 ---
-{{< datengarten-infobox >}}
 
 GLORIA PILOT MISSION PROPOSAL
 -----------------------------