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/34.md

@@ -5,14 +5,15 @@ no: 34
 title: "Datengarten 34"
 subtitle: "The social web"
 date: 2009-10-22T00:00:00+02:00
-event_date: 2009-10-22T00:00:00+02:00
+event:
+  start: 2009-10-22T00:00:00+02:00
+  end:   2009-10-22T00:00:00+02:00
 location: CCCB
 speaker: Henry Story
 speaker_url:
-language: Englisch
+language: en
 streaming: false
 ---
-{{< datengarten-infobox >}}
 
 **How to free yourself of your social networks and create a global community**