Improve registrar area linked user switch

- Introduce profile
- Move linked users to profile
- Use PUT

#599

app/controllers/registrar/profile_controller.rb

@@ -0,0 +1,17 @@
+class Registrar
+  class ProfileController < BaseController
+    skip_authorization_check
+
+    helper_method :linked_users
+
+    def show
+      @user = current_user
+    end
+
+    private
+
+    def linked_users
+      current_user.linked_users
+    end
+  end
+end