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/75.md

@@ -6,12 +6,13 @@ no: 75
 subtitle: "Structural Patterns in the Rise of Germany’s New Right on Facebook"
 speaker: Sebastian
 date: 2017-03-14T00:00:00+02:00
-event_date: 2017-03-14T20:00:00+02:00
+event:
+  start: 2017-03-14T20:00:00+02:00
+  end:   2017-03-14T22:00:00+02:00
 location: CCCB
-language: Englisch
+language: en
 streaming: false
 ---
-{{< datengarten-infobox >}}
 
 In recent years, Germany has experienced a massive rise in popularity of
 the so-called 'New Right', both in the form of political parties