Fixes no validation on category dropdown which causes errors

If you add or edit a product and don't choose a category, you could still add the product. This caused a Spoon exception when click on the product. The validation on the category_id dropdown didn't seem to be working.

I used the category validation from by the blog module. The "none" category gets a special index and the validation checks if this index is active. 

I also added a few lines of css to make the dropdown full width in the sidebar (just like the blog module does). And I noticed that the multi-selection box of related products was only 20px wide when I had only 1 product with a short title. So I also made this box take the full width of the sidebar.