Use body element as dropdownParent for tomselect elements

This improves UX

assets/controllers/elements/attachment_autocomplete_controller.js

@@ -42,6 +42,7 @@ export default class extends Controller {
             selectOnTab: true,
             //This a an ugly solution to disable the delimiter parsing of the TomSelect plugin
             delimiter: 'VERY_L0NG_D€LIMITER_WHICH_WILL_NEVER_BE_ENCOUNTERED_IN_A_STRING',
+            dropdownParent: 'body',
             render: {
                 item: (data, escape) => {
                     return '<span>' + escape(data.label) + '</span>';