organize beta warning

resource/locales/en-US/translation.json

@@ -788,7 +788,6 @@
     "rebuild_description_3":"This may take a while."
   },
   "export_management": {
-    "beta_warning": "This function is Beta.",
     "exporting_data_list": "Exporting Data List",
     "exported_data_list": "Exported Data List",
     "export_collections": "Export Collections",

resource/locales/ja/translation.json

@@ -773,7 +773,6 @@
     "rebuild_description_3":""
   },
   "export_management": {
-    "beta_warning": "この機能はベータ版です",
     "exporting_data_list": "エクスポート中のデータ",
     "exported_data_list": "エクスポートデータリスト",
     "export_collections": "コレクションのエクスポート",

src/client/js/components/Admin/Import/GrowiZipImportSection.jsx

@@ -75,6 +75,10 @@ class GrowiZipImportSection extends React.Component {
       <Fragment>
         <h2>{t('importer_management.import_form_growi')}</h2>
 
+        <div className="alert alert-warning">
+          <i className="icon-exclamation"></i> { t('importer_management.beta_warning') }
+        </div>
+
         {this.state.fileName ? (
           <div className="px-4">
             <GrowiZipImportForm