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/51.md

@@ -5,15 +5,16 @@ title: "Datengarten 51"
 no: 51
 subtitle: "Puppet … Konfigurationsmanagement"
 date: 2015-05-12T00:00:00+02:00
-event_date: 2015-05-12T20:00:00+02:00
+event:
+  start: 2015-05-12T20:00:00+02:00
+  end:   2015-05-12T22:00:00+02:00
 location: CCCB
 speaker: Robert
 speaker_url: https://twitter.com/zero\_0ne
-language: Deutsch
+language: de
 streaming: yes
 recording: https://media.ccc.de/v/dg51-puppet
 ---
-{{< datengarten-infobox >}}
 
 "Puppet … Konfigurationsmanagement“ (KM; englisch configuration
 management, CM) ist eine Managementdisziplin, die organisatorische und