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.
Won't using `#delete` here mutate the original value, getting rid of the `:type` key from the attributes forever? `#role_attributes_from` seems to return the original rather than a new version each time it's called.

I think avoiding mutation is generally safer. Rails provides `#except` and `#slice` methods which make this simpler, eg:

```ruby
self.attributes = role_attributes.except(:type)
```