js: Normalize strings to double quotes.

Prettier would do this anyway, but it’s separated out for a more
reviewable diff.  Generated by ESLint.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/js/i18n.js

@@ -1,7 +1,7 @@
-import i18next from 'i18next';
+import i18next from "i18next";
 
 i18next.init({
-    lng: 'lang',
+    lng: "lang",
     resources: {
         lang: {
             translation: page_params.translation_data,