Ajaxify node rendering in calendar colorbox

#5433 
Currently (3.10, 3.11 and 3.x) each os_events view render all nodes in the view, so the colorbox will have them without extra requests. This cause long loading time.
To avoid it, we AJAXify the node loading, while still using colorbox. The simple quick solution is to use colorbox_node, but in the end we can grab just the code we want.

Result is reduce of 58% in time per request for www/economics/calendar (on local server).
Before: Time per request:       11397.553 [ms] (mean)
After: Time per request:       4893.478 [ms] (mean)

The is one difference between the old solution and this one - there are no prev/next buttons inside the colorbox between events. If this a critical feature, we will find a way to put them back.