Remove pointer events on the entire UI when a dropdown menu is open (#6648)

* Remove pointer events on the entire UI when a dropdown menu is open

This prevents operations to change the location of the menu such as
scrolling.

* Fix mistake from merge

app/javascript/mastodon/actions/dropdown_menu.js

@@ -0,0 +1,10 @@
+export const DROPDOWN_MENU_OPEN = 'DROPDOWN_MENU_OPEN';
+export const DROPDOWN_MENU_CLOSE = 'DROPDOWN_MENU_CLOSE';
+
+export function openDropdownMenu(id, placement) {
+  return { type: DROPDOWN_MENU_OPEN, id, placement };
+}
+
+export function closeDropdownMenu(id) {
+  return { type: DROPDOWN_MENU_CLOSE, id };
+}