Admins can create categories via form

https://trello.com/c/Wn1dGnlj/32-as-an-admin-i-can-create-categories

![image](https://cloud.githubusercontent.com/assets/8681076/4982381/51fb7324-6912-11e4-8a79-008624f84171.png)

This can just be `"form"` instead of `"categories/form"` since this view is in `categories` too.