Highlight erroneous fields by adding a class to them

The default rails scheme for highlighting errors is to wrap the
field in a div, but that changes the structure of the page and can
change the meaning of CSS rules applied to the fields.

As an alternative we now apply a class to the fields, and use that
in the CSS to apply a highlight.

config/initializers/field_error.rb

@@ -0,0 +1,9 @@
+ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
+  class_attr_index = html_tag.index 'class="'
+
+  if class_attr_index
+    html_tag.insert class_attr_index+7, 'field_with_errors '
+  else
+    html_tag.insert html_tag.index(/\/?>/), ' class="field_with_errors"'
+  end
+end