Fix page incorrectly scrolling when bringing up dropdown menus (#13574)

Fixes #13573

For some reason (I suspect this may be related to focusing the item before it
got drown by the browser), Firefox scrolls to top when bringing up dropdown
menus with pre-selected items.

This commit uses the “preventScroll” option as, due to the placement behavior,
the menu should be visible anyway and not trigger scrolling.

app/javascript/mastodon/components/dropdown_menu.js

@@ -46,7 +46,7 @@ class DropdownMenu extends React.PureComponent {
     document.addEventListener('keydown', this.handleKeyDown, false);
     document.addEventListener('touchend', this.handleDocumentClick, listenerOptions);
     if (this.focusedItem && this.props.openedViaKeyboard) {
-      this.focusedItem.focus();
+      this.focusedItem.focus({ preventScroll: true });
     }
     this.setState({ mounted: true });
   }