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/permissions.js

@@ -0,0 +1,12 @@
+'use strict';
+
+import {getUrl} from "./urls";
+import axios from "./axios";
+
+async function checkPermissions(request) {
+    return await axios.post(getUrl('rest/permissions-check'), request);
+}
+
+export {
+    checkPermissions
+}