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/73.md

@@ -7,13 +7,14 @@ subtitle: "Hacking with Care - Tactical and ethical care for all"
 speaker: Jérémie Zimmermann
 speaker_url: https://twitter.com/hackingwithcare
 date: 2017-01-10T00:00:00+02:00
-event_date: 2017-01-10T20:00:00+02:00
+event:
+  start: 2017-01-10T20:00:00+02:00
+  end:   2017-01-10T22:00:00+02:00
 location: CCCB
-language: Englisch
+language: en
 streaming: true
 recording: https://media.ccc.de/v/dg-73
 ---
-{{< datengarten-infobox >}}
 
 We see everyday members of our communities -especially ones under high
 pressure- go burn-out, depressed, or worse. Caring about ourselves and