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/view.php

@@ -60,6 +60,9 @@
         require_login();
     }
 
+    // Initialize the session variables
+    calendar_session_vars();
+
     //add_to_log($course->id, "course", "view", "view.php?id=$course->id", "$course->id");
     $now = usergetdate(time());
     $pagetitle = '';