fix(Highlighters): ensure highlighter is turned off on disable

When the interactable object is disabled, the highlighter should also
be turned off otherwise when the object is re-enabled it still has
the highlighted colour even if it's not being touched.

Assets/VRTK/Scripts/Interactions/Highlighters/VRTK_BaseHighlighter.cs

@@ -87,5 +87,10 @@ public static VRTK_BaseHighlighter GetActiveHighlighter(GameObject obj)
 
             return objectHighlighter;
         }
+
+        protected virtual void OnDisable()
+        {
+            Unhighlight();
+        }
     }
 }