calendar: performance optimisations + behaviour fix (MDL-7416 MDL-9617)

With 2500 courses the calendar was causing around 15000 queries at login and 5000 on the home page.

- Replaced queries inside loops with single unlooped queries.
- Fixed calendar so that course events will show when the users role is assigned at a category level.
- Moved calendar_session_vars() function call out of calendar/lib.php (including a lib should not have side effects)

Author: Matt Clarkson - with some conflict resolution from MartinL

calendar/set.php

@@ -49,6 +49,9 @@
     $cal_m = optional_param('cal_m');
     $cal_y = optional_param('cal_y');
 
+    // Initialize the session variables
+    calendar_session_vars();
+
     switch($var) {
         case 'setuser':
             // Not implemented yet (or possibly at all)