/category/add: DuplicateKeyException when name contains mix of spaces and hyphens

[php-coder]

Yet another one of consequence of #206: after denying to specify multiple hyphens in #465 it's still possible to have DuplicateKeyException:

• create category with name te-st
• after that try to create another category with name te- st

We should add yet another validator that takes name, convert it to slug and checks in database whether it exist or not.