Update the form_tag helper to comply with CSP

Content Security Policy does not allow inline style attributes for HTML
tags. The actionview form helper added an inline style attribute to the
div within the form containing hidden fields added by the helper. I
replaced the style attribute with a class attribute named "extra_tags."
I defined this class in the scaffold stylesheet generator and set its
style attributes to match the inline style attributes that were
previously being added to the div generated by the helper.