Bug 1382538: Run FTL translation through FluentSerializer

Before writing translation submitted via Pontoon to the DB, run them
through the FluentSerializer for consistency with Python Serializer.
That way translation doesn't get reimported on next sync, because it
appears exactly the same when read from the DB and VCS.

pontoon/base/static/js/fluent_interface.js

@@ -1,6 +1,7 @@
 /* Public functions used across different files */
 var Pontoon = (function (my) {
   const fluentParser = new FluentSyntax.FluentParser({ withSpans: false });
+  const fluentSerializer = new FluentSyntax.FluentSerializer();
 
   return $.extend(true, my, {
     fluent: {
@@ -277,7 +278,8 @@ var Pontoon = (function (my) {
           translation = ' = ' + translation;
         }
 
-        return entity.key + translation + '\n';
+        var content = entity.key + translation;
+        return fluentSerializer.serializeEntry(fluentParser.parseEntry(content));
       },