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/32.md

@@ -5,7 +5,9 @@ no: 32
 title: "Datengarten 32"
 subtitle: "Liquid Democracy"
 date: 2018-09-03T00:00:00+02:00
-event_date: 2018-09-03T20:00:00+02:00
+event:
+  start: 2018-09-03T20:00:00+02:00
+  end:   2018-09-03T22:00:00+02:00
 location: CCCB
 speaker: Martin Häcker & Daniel Reichert
 speaker_url:
@@ -13,7 +15,6 @@ language:
 streaming:
 recording:
 ---
-{{< datengarten-infobox >}}
 
 **Direkter Parlamentarismus – gemeinsam verbindlich entscheiden** (in
 deutscher Sprache)