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/56.md

@@ -5,14 +5,15 @@ title: "Datengarten 56"
 no: 56
 subtitle: "Hands-on Rad1o"
 date: 2015-09-08T00:00:00+02:00
-event_date: 2015-09-08T20:00:00+02:00
+event:
+  start: 2015-09-08T20:00:00+02:00
+  end:   2015-09-08T22:00:00+02:00
 location: CCCB
 speaker: schneider
-language: Deutsch
+language: de
 streaming: yes
 recording: https://media.ccc.de/v/dg56-Hands-on_Rad1o
 ---
-{{< datengarten-infobox >}}
 
 Der Datengarten 56 widmet sich voll und ganz dem
 [Rad1o](https://rad1o.badge.events.ccc.de/) Badge vom [CCCamp