Fixed sandbox. Multiple tabs work now.

WiP on selectable mosaico templates.

TODO: Make files always point to trusted URL, such that we don't have to rebase them. They are public anyway. The same goes for mosaico endpoints: /mosaico/templates and /mosaico/img

client/src/lib/i18n.js

@@ -2,6 +2,7 @@ import i18n from 'i18next';
 import XHR from 'i18next-xhr-backend';
 // import Cache from 'i18next-localstorage-cache';
 import mailtrainConfig from 'mailtrainConfig';
+import {getUrl} from "./urls";
 
 i18n
     .use(XHR)
@@ -23,6 +24,10 @@ i18n
 
         interpolation: {
             escapeValue: false // not needed for react
+        },
+
+        backend: {
+            loadPath: getUrl('locales/{{lng}}/{{ns}}.json')
         }
     });