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/45.md

@@ -5,13 +5,14 @@ title: Datengarten 45
 no: 45
 subtitle: "Aaadhaar: What kind of 'database state' is the Indian government creating?"
 date: 2014-07-04T00:00:00+02:00
-event_date: 2014-07-04T20:00:00+02:00
+event:
+  start: 2014-07-04T20:00:00+02:00
+  end:   2014-07-04T22:00:00+02:00
 location: CCCB
 speaker: Sumandro Chattapadhyay
-language: Englisch
+language: en
 streaming: no
 ---
-{{< datengarten-infobox >}}
 
 A discussion of the Aadhaar project as a digital identity platform for
 governance