Change /buyers route to use request args
Change /buyers route to use request args

The /buyers now gets the brief_id from the request arguments, as that's
how it's provided by the form on the index page.

alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend