Design fixes for Add/Edit Product CPT (v3)

Replacement re-based PR which supersedes #730 for issue #391

Still to do. (Please add if you spot anything):

- [x] There's also a lot of underscore selector hacks which I think are just for IE6. Remove these? See http://stackoverflow.com/questions/15641506/css-underscore-hack-for-ie-still-relevant
- [ ] Check over in both WP 3.8 and 3.7. Target earlier versions with fixes, not newer versions. See http://make.wordpress.org/ui/2013/11/19/targeting-the-new-dashboard-design-in-a-post-mp6-world/
- [x] Remove grey background behind variation panel tabs.
- [x] Add back table bottom border in WP3.8 for Download tab table. Overwrite for 3.7 if required.
- [x] Needs rebasing again
- [x] Ensure strings are en_US by default
- [ ] Remove inline JS when possible.
- [ ] Follow WordPress conventions for JS.
- [ ] Don't assign HTML to a variable unless absolutely necessary.