Drop database default for role type.
Move setting of role type attributes into model.

This simplifies the controller significantly and makes the presenter only
responsible for creating the values for the select box. Also remove default
values from presenter.
Simplify form partial.

Remove `becomes` which changes the type of the passed-in Role.
Add url helper to return generic role admin path.
Fix test after role type changes.

Most breakages were due to the removal of `becomes` in the view: now we need to
specify the Role subclass specifically.
Move ministerial roles to end of options.
Fix unit and feature tests.
Get correct role_type value in accessor.

This removes the need for the helper.
Merge pull request #1608 from alphagov/default_role

Remove Cabinet Minister default when creating roles