Adding react scrollbar to the RHS thread (#2693)

* Adding scrollbar to the RHS thread

* Fixing the RHS thread scrolling issue

webapp/sass/components/_scrollbar.scss

@@ -22,4 +22,10 @@ body {
     scrollbar-highlight-color: #7D7E94;
     scrollbar-shadow-color: #2D2C4D;
     scrollbar-track-color: rgba(0, 0, 0, .1);
-}
+}
+
+.scrollbar--horizontal,
+.scrollbar--vertical {
+    @include border-radius(2px);
+    @include alpha-property(background-color, $black, .5);
+}