Add a row to a product type's "Manage fields" form for attribute fields

Something like this.

One thought is we can use a GET parameter in the URL or something and use it to alter the field edit form so it has to be required and single value. On subsequent edits, users should be able to change these, but we can just have a general warning displayed that "field_x no longer qualifies as a product attribute field because you changed setting_y."

This still needs to be done, but I've added a separate issue for another aspect of this usability that needs to be improved: #1038320: Replace the "magic configuration" of attribute fields with explicit settings

As it turns out, the issue I linked in #7 above evolved from a simple bypass option to an explicit group of settings enabling a field as an attribute field and adjusting its representation on the Add to Cart form. As such, I don't think we need to pursue this particular issue any further. I've flagged #1038320: Replace the "magic configuration" of attribute fields with explicit settings for review at the upcoming UX sprint, but I believe it will be a better approach than the magic configuration w/ a separate "add field" row on the manage fields tab.