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/74.md

@@ -7,13 +7,14 @@ subtitle: "Bitcoin: Privacy and Censorship Resistance Beyond the Web"
 speaker: Matt Corallo
 speaker_url: https://twitter.com/TheBlueMatt
 date: 2017-02-14T00:00:00+02:00
-event_date: 2017-02-14T20:00:00+02:00
+event:
+  start: 2017-02-14T20:00:00+02:00
+  end:   2017-02-14T22:00:00+02:00
 location: CCCB
-language: Englisch
+language: en
 streaming: true
 recording: https://media.ccc.de/v/dg-74
 ---
-{{< datengarten-infobox >}}
 
 In much of the western world, governments have developed the ability to
 prevent individuals or organizations from accessing financial services,