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/52.md

@@ -5,14 +5,15 @@ title: "Datengarten 52"
 no: 52
 subtitle: "Reproducible Builds"
 date: 2015-05-26T00:00:00+02:00
-event_date: 2015-05-26T20:00:00+02:00
+event:
+  start: 2015-05-26T20:00:00+02:00
+  end:   2015-05-26T22:00:00+02:00
 location: CCCB
 speaker: h01ger
-language: Deutsch
+language: de
 streaming: yes
 recording: https://media.ccc.de/v/dg52-reproducablebuilds
 ---
-{{< datengarten-infobox >}}
 
 With free software, anyone can inspect the source code for malicious
 flaws. But Debian provide binary packages to its users. The idea of