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/reducers/index.js

@@ -1,4 +1,5 @@
 import { combineReducers } from 'redux-immutable';
+import dropdown_menu from './dropdown_menu';
 import timelines from './timelines';
 import meta from './meta';
 import alerts from './alerts';
@@ -26,6 +27,7 @@ import lists from './lists';
 import listEditor from './list_editor';
 
 const reducers = {
+  dropdown_menu,
   timelines,
   meta,
   alerts,