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

layouts/datengarten/section.ics

@@ -0,0 +1,20 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//cccb//datengarten calendar//EN
+CALSCALE:GREGORIAN
+METHOD:PUBLISH
+{{range .Pages -}}
+BEGIN:VEVENT
+ORGANIZER;CN="CCCB":mailto:datengarten@berlin.ccc.de
+SUMMARY:{{.Title}}{{with .Params.subtitle}}: {{.}}{{end}}
+UID:{{.Params.no}}@datengarten.berlin.ccc.de
+SEQUENCE:0
+STATUS:CONFIRMED
+DTSTAMP:{{dateFormat "20060102T150405Z" .Params.event.start}}
+DTSTART;TZID=Europe/Berlin:{{dateFormat "20060102T150405" .Params.event.start}}
+DTEND;TZID=Europe/Berlin:{{dateFormat "20060102T150405" .Params.event.end}}
+LOCATION:{{with .Params.location}}{{.}}{{else}}CCCB{{end}}
+URL:{{.Permalink}}
+END:VEVENT
+{{end -}}
+END:VCALENDAR