[misc bugfix] Remove body width & scroll jumping occurring in Chrome

- use `react-focus-on`'s scrollLock API to prevent this (they manage preserving the width of the scrollbar for us)

src/components/modal/modal.tsx

@@ -91,7 +91,7 @@ export const EuiModal: FunctionComponent<EuiModalProps> = ({
 
   return (
     <EuiOverlayMask>
-      <EuiFocusTrap initialFocus={initialFocus}>
+      <EuiFocusTrap initialFocus={initialFocus} scrollLock>
         {
           // Create a child div instead of applying these props directly to FocusTrap, or else
           // fallbackFocus won't work.