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/77.md

@@ -7,13 +7,14 @@ subtitle: "Calculate your Bus Factor - a practical introduction to GNU R"
 speaker: Robin Gower
 speaker_url: https://twitter.com/robsteranium
 date: 2017-05-09T00:00:00+02:00
-event_date: 2017-05-09T20:00:00+02:00
+event:
+  start: 2017-05-09T20:00:00+02:00
+  end:   2017-05-09T22:00:00+02:00
 location: CCCB
-language: Englisch
+language: en
 streaming: true
 recording: https://media.ccc.de/v/dg-77
 ---
-{{< datengarten-infobox >}}
 
 The strength of open source comes from the community, but what would happen if some of it were to go missing? This talk will show you one way of measuring the sustainability of the projects you love (and perhaps depend upon). We'll get our data from git commit logs and analyse it using GNU-R, the statistical language and computing environment.