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/66.md

@@ -6,13 +6,14 @@ no: 66
 subtitle: "Introduction to W3C & Securing the Open Web Platform"
 speaker: "Felix Sasaki, Georg Rehm, Wendy Seltzer"
 date: 2016-05-10T00:00:00+02:00
-event_date: 2016-05-10T20:00:00+02:00
+event:
+  start: 2016-05-10T20:00:00+02:00
+  end:   2016-05-10T22:00:00+02:00
 location: CCCB
-language: Englisch
+language: en
 streaming: true
 recording: https://media.ccc.de/v/dg66
 ---
-{{< datengarten-infobox >}}
 
 #Part 1: A brief introduction to the W3C