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_rds.txt

@@ -0,0 +1,5 @@
+<TMPL_IF use_client_cache>Cache-Control: max-age=600, must-revalidate
+</TMPL_IF>Content-type:text/plain; charset=UTF-8;
+Access-Control-Allow-Origin: *
+
+<TMPL_LOOP NAME=events><TMPL_IF __first__><TMPL_VAR rds_title></TMPL_IF></TMPL_LOOP>