3087 Forgot the edit file for the skins form

https://code.google.com/p/otwarchive/issues/detail?id=3087

Skins advanced section was still broken when editing because I forgot the edit file, which also has the form_form
(To head any questions off at the pass, since I will be afk shortly, the class _could_ be edit instead of post, but the logic behind that, courtesy of my little document for a form class system: "We use this rather than separate `create` and `edit` classes because most forms that are used for creating are also used for editing, such as the standard work form.")