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_utc_time.json

@@ -0,0 +1,14 @@
+<TMPL_IF use_client_cache>Cache-Control: max-age=600, must-revalidate
+</TMPL_IF>Content-type:application/json; charset=utf-8;
+Access-Control-Allow-Origin: *
+
+{
+"events" : [
+<TMPL_LOOP NAME=events>{
+    "start" : "<TMPL_VAR start_utc_epoch>"
+    ,"end"   : "<TMPL_VAR end_utc_epoch>"
+    <TMPL_IF series_name> ,"series_name" : "<TMPL_VAR series_name>"</TMPL_IF>
+    <TMPL_IF title> ,"title" : "<TMPL_VAR title>"</TMPL_IF>
+}<TMPL_UNLESS __last__>,</TMPL_UNLESS></TMPL_LOOP>
+    ]
+}