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/preferences.php

@@ -13,7 +13,9 @@
     if ($course->id != SITEID) {
         require_login($course->id);
     }
-
+    // Initialize the session variables
+    calendar_session_vars();
+    
 /// If data submitted, then process and store.
 
     if ($form = data_submitted()) {