We don't need any row classes anymore because the <body> already has a
maximum width. As for columns, we only have one column in this form, so
we don't need them either. Besides, the form's parent element already
has a padding.
In general, we always use relative URLs (using `_path`), but sometimes
we were accidentally using absolute URLs (using `_url`). It's been
reported i might cause some isuses if accepting both HTTP and HTTPS
connections, although we've never seen the case.
In any case, this change makes the code more consistent and makes the
generated HTML cleaner.
