Add field serialization in /admin/fields put request.

Instead of returning {'result': 'ok'}, return a more appropriate summary of the
fields that have been modified as they do appear on the database after the
edit.
Remove the default argument language in controller functions for admin/fields.

For two reasons:
1. I want them to fail if I haven't provided a language argument from the
request.
2. At the moment they are never called with an implicit argument.