There are several interfaces where we have the option to add a new item and rearrange existing items in the same place. On the user roles page the "Add role" button is waay off to the right, and is easily overlooked. On the field UI the "Save" button both adds a new role and saves the order.
Let's implement the pattern from the Field UI form on the Roles page too.
Also related: https://github.com/backdrop/backdrop-issues/issues/927