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/57.md

@@ -5,15 +5,16 @@ title: "Datengarten 57 Spezial"
 no: 57
 subtitle: "From Secret Manoeuvres to Undercover Research"
 date: 2015-09-28T00:00:00+02:00
-event_date: 2015-09-28T20:00:00+02:00
+event:
+  start: 2015-09-28T20:00:00+02:00
+  end:   2015-09-28T22:00:00+02:00
 location: c-base, Rungestrasse 20, 10179 Berlin
 speaker: Eveline Lubbers
 speaker_url:
-language: Englisch
+language: en
 streaming: true
 recording: https://media.ccc.de/v/dg57_From_Secret_Manoeuvres_to_Undercover_Research
 ---
-{{< datengarten-infobox >}}
 
 Beschreibung Deutsch
 --------------------