MDL-34862 question preview: improve preview ownership check.

Users should only be able to access their own quetion preview. In the
past, for reasons I can no longer remember, this was enforced
using the session. It is much better to set the question_usage to belong
to the user's context.

question/engine/questionusage.php

@@ -63,7 +63,7 @@ class question_usage_by_activity {
      */
     protected $preferredbehaviour = null;
 
-    /** @var object the context this usage belongs to. */
+    /** @var context the context this usage belongs to. */
     protected $context;
 
     /** @var string plugin name of the plugin this usage belongs to. */
@@ -104,7 +104,7 @@ class question_usage_by_activity {
         return $this->preferredbehaviour;
     }
 
-    /** @return object the context this usage belongs to. */
+    /** @return context the context this usage belongs to. */
     public function get_owning_context() {
         return $this->context;
     }