dropbox: add scopes to oauth request and optionally "members.read"

This change adds the scopes rclone wants during the oauth request.
Previously rclone left these blank to get a default set.

This allows rclone to add the "members.read" scope which is necessary
for "impersonate" to work, but only when it is in use as it require
authorisation from a Team Admin.

See: https://forum.rclone.org/t/dropbox-no-members-read/22223/3

lib/oauthutil/oauthutil.go

@@ -639,6 +639,7 @@ func (s *authServer) Init() error {
 			http.Error(w, "State did not match - please try again", http.StatusForbidden)
 			return
 		}
+		fs.Debugf(nil, "Redirecting browser to: %s", s.authURL)
 		http.Redirect(w, req, s.authURL, http.StatusTemporaryRedirect)
 		return
 	})