Create buyers view and add find by brief id route
Create buyers view and add find by brief id route

There is no view file for viewing buyers, so this creates one. It
includes a route which finds a brief via the api client and renders a
template with the associated users included.

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