Remove FormFor Helper

The `FormFor` patch in HAML is supposed to add proper indentation to the rendered form. This is already (correctly) implemented in HAML's `form_tag` patch. Since `#form_for` internally uses `#form_tag` there is no need for redundancy that introduced a bug.

This bug happened to render the form's fields (actually, _any_ form markup) before the form instead of within the tags. To make it even more complicated, this only is visible when changing the view context in Rails.

Anyway, this is fixed and tested now :grin: