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/31.md

@@ -5,13 +5,14 @@ no: 31
 title: "Datengarten 31"
 subtitle: "Dem Compiler beim Optimieren zuschauen"
 date: 2009-06-25T00:00:00+02:00
-event_date: 2009-06-25T20:00:00+02:00
+event:
+  start: 2009-06-25T20:00:00+02:00
+  end:   2009-06-25T22:00:00+02:00
 location: CCCB
 speaker: hannes
-language: Deutsch
+language: de
 streaming: false
 ---
-{{< datengarten-infobox >}}
 
 **A tool for debugging compiler optimizations and type inference** (in
 deutscher Sprache)