template: custom themes for agenda

For agenda a custom theme can be configured in locations/theme.
Templates are read from the subdirectory "templates/<theme>/".  The
default theme is "default".  All Templates are moved to
templates/default.

website/agenda/templates/default/event.ics

@@ -0,0 +1,49 @@
+<TMPL_IF use_client_cache>Cache-Control: max-age=600, must-revalidate
+</TMPL_IF>Content-Type:text/calendar; charset="UTF-8"
+Content-Disposition: attachment; filename="<TMPL_VAR project_title>-programm.ics"
+Access-Control-Allow-Origin: *
+
+BEGIN:VCALENDAR
+PRODID:-//calcms//DE
+VERSION:2.0
+CALSCALE:GREGORIAN
+METHOD:PUBLISH
+X-WR-CALNAME:<TMPL_VAR project_title> Programm (ical)
+X-WR-TIMEZONE:<TMPL_VAR time_zone>
+X-WR-CALDESC:
+BEGIN:VTIMEZONE
+TZID:<TMPL_VAR time_zone>
+X-LIC-LOCATION:<TMPL_VAR time_zone>
+BEGIN:DAYLIGHT
+TZOFFSETFROM:+0100
+TZOFFSETTO:+0200
+TZNAME:CEST
+DTSTART:19700329T020000
+RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU
+END:DAYLIGHT
+BEGIN:STANDARD
+TZOFFSETFROM:+0200
+TZOFFSETTO:+0100
+TZNAME:CET
+DTSTART:19701025T030000
+RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU
+END:STANDARD
+END:VTIMEZONE
+<TMPL_LOOP NAME=events><TMPL_IF NAME=dtstart>BEGIN:VEVENT
+DTSTART;TZID=<TMPL_VAR time_zone>:<TMPL_VAR NAME=dtstart>
+<TMPL_IF NAME=dtend>DTEND;TZID=<TMPL_VAR time_zone>:<TMPL_VAR NAME=dtend>
+</TMPL_IF>DTSTAMP;TZID=<TMPL_VAR time_zone>:<TMPL_VAR modified_at>
+UID:<TMPL_VAR NAME=event_id>
+CLASS:PUBLIC
+CREATED;TZID=<TMPL_VAR time_zone>:<TMPL_VAR created_at>
+DESCRIPTION:<TMPL_VAR NAME=content_ical>
+LAST-MODIFIED;TZID=<TMPL_VAR time_zone>:<TMPL_VAR modified_at>
+LOCATION:<TMPL_VAR location_mapped>
+SEQUENCE:0
+STATUS:CONFIRMED
+SUMMARY:<TMPL_VAR full_title>
+URL:<TMPL_VAR cache_base_url>/<TMPL_VAR .controllers.event>/<TMPL_VAR event_id>.html#<TMPL_VAR event_uri>
+TRANSP:OPAQUE
+CATEGORIES:<TMPL_VAR project_title>
+END:VEVENT
+</TMPL_IF></TMPL_LOOP>END:VCALENDAR