Deny HTML5 permissions when question is cancelled.

Before we just ignored the request but deleted the Question object, which lead
to a RuntimeError when the question was cancelled and the page was reloaded,
because on_permission_canceled tried to abort the question.

qutebrowser/browser/webpage.py

@@ -316,6 +316,7 @@ class BrowserPage(QWebPage):
                 self.setFeaturePermission, frame, feature,
                 QWebPage.PermissionDeniedByUser)
             q.answered_no.connect(no_action)
+            q.cancelled.connect(no_action)
 
             q.completed.connect(q.deleteLater)