update filtered_search to ES module syntax

app/assets/javascripts/filtered_search/dropdown_hint.js

 import Filter from '~/droplab/plugins/filter';
-
-require('./filtered_search_dropdown');
+import './filtered_search_dropdown';
 
 class DropdownHint extends gl.FilteredSearchDropdown {
   constructor(droplab, dropdown, input, filter) {

app/assets/javascripts/filtered_search/dropdown_non_user.js

@@ -2,8 +2,7 @@
 
 import Ajax from '~/droplab/plugins/ajax';
 import Filter from '~/droplab/plugins/filter';
-
-require('./filtered_search_dropdown');
+import './filtered_search_dropdown';
 
 class DropdownNonUser extends gl.FilteredSearchDropdown {
   constructor(droplab, dropdown, input, filter, endpoint, symbol) {

app/assets/javascripts/filtered_search/dropdown_user.js

 /* global Flash */
 
 import AjaxFilter from '~/droplab/plugins/ajax_filter';
-
-require('./filtered_search_dropdown');
+import './filtered_search_dropdown';
 
 class DropdownUser extends gl.FilteredSearchDropdown {
   constructor(droplab, dropdown, input, filter) {

app/assets/javascripts/filtered_search/filtered_search_bundle.js

-require('./dropdown_hint');
-require('./dropdown_non_user');
-require('./dropdown_user');
-require('./dropdown_utils');
-require('./filtered_search_dropdown_manager');
-require('./filtered_search_dropdown');
-require('./filtered_search_manager');
-require('./filtered_search_token_keys');
-require('./filtered_search_tokenizer');
-require('./filtered_search_visual_tokens');
+import './dropdown_hint';
+import './dropdown_non_user';
+import './dropdown_user';
+import './dropdown_utils';
+import './filtered_search_dropdown_manager';
+import './filtered_search_dropdown';
+import './filtered_search_manager';
+import './filtered_search_token_keys';
+import './filtered_search_tokenizer';
+import './filtered_search_visual_tokens';

app/assets/javascripts/filtered_search/filtered_search_tokenizer.js

-require('./filtered_search_token_keys');
+import './filtered_search_token_keys';
 
 class FilteredSearchTokenizer {
   static processTokens(input) {