fix(form): avoid additional margin with nested fields

When there is a fields class in another fields class, an extra margin-bottom is added.

Thanks to @dutrieux , who already provided the fix in the related issue 😄

Closes #1202

src/definitions/collections/form.less

@@ -47,6 +47,7 @@
   margin: @fieldMargin;
 }
 
+.ui.form .fields .fields,
 .ui.form .field:last-child,
 .ui.form .fields:last-child .field {
   margin-bottom: 0;