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.
Making step number implicit.

* Implement in models new() for step so that number is no more required, and
  auto-increments itself (I found *NO* way to perform this in storm or sqlite
  keeping the same primary keys)
* Add unittests for those changes.