Remove Cabinet Minister default when creating roles

Show a blank default in the role type select when creating a new role, and validate that user specifically chooses an option. Move ministerial options to end.
Inconsistent use of brackets on these lines. After you've deleted the STI `type` param above, you can simply assign the rest with `attributes=`, i.e.

```ruby
self.type = role_attributes.delete(:type)
self.attributes = role_attributes
```