fix(toolbar): let click through gradient (#4107)

Otherwise it can eat clicks on elements it is above,
like YouTube and Etherpad controls.

css/_toolbars.scss

@@ -48,6 +48,7 @@
         height: 160px;
         width: 100%;
         bottom: -160px;
+        pointer-events: none;
         position: absolute;
         z-index: $toolbarBackgroundZ;
     }