Disable link loading animation when offscreen

Stop the link preview loading indicator animation running when the delegate isn't visible.

CCBUG: 465715

src/qml/Component/Timeline/MessageDelegate.qml

@@ -31,6 +31,7 @@ TimelineContainer {
         }
         LinkPreviewDelegate {
             Layout.fillWidth: true
+            indicatorEnabled: messageDelegate.isVisibleInTimeline()
         }
     }
 }