fix: various dropdown problems (#38020)

1. remove legacy onResponseKeepSelectedItem, refactor the code to
dropdown.js
2. make dropdown correctly handle "single selection + remote query + filter"
    * fix #38018
3. fix incorrect "transition" class usage for the dropdown dividers

web_src/js/features/repo-new.ts

@@ -47,7 +47,6 @@ function initRepoNewTemplateSearch(form: HTMLFormElement) {
               value: String(tmplRepo.repository.id),
             });
           }
-          $repoTemplateDropdown.fomanticExt.onResponseKeepSelectedItem($repoTemplateDropdown, inputRepoTemplate.value);
           return {results};
         },
         cache: false,