Move permission helper method closer to where it's used

The `BatchInvitationPermissionsHelper#formatted_permission_name` method
is only used in `UsersHelper#items_for_permission_checkboxes` and so it
makes more sense to define it in `UsersHelper` closer to where it's
being used. We _could_ also make it private, but I decided it was worth
keeping the tests around it and so left it as public.

app/helpers/users_helper.rb

@@ -88,4 +88,12 @@ def items_for_permission_checkboxes(application:, user: nil)
   def supported_permission_checkbox_id(application, permission)
     "user_application_#{application.id}_supported_permission_#{permission.id}"
   end
+
+  def formatted_permission_name(application_name, permission_name)
+    if permission_name == SupportedPermission::SIGNIN_NAME
+      "Has access to #{application_name}?"
+    else
+      permission_name
+    end
+  end
 end

test/helpers/users_helper_test.rb

@@ -74,8 +74,6 @@ class UsersHelperTest < ActionView::TestCase
     end
   end
 
-  include BatchInvitationPermissionsHelper
-
   context "#items_for_permission_checkboxes" do
     should "return permission options suitable for checkboxes component" do
       application = create(:application)
@@ -114,4 +112,14 @@ class UsersHelperTest < ActionView::TestCase
       assert_equal expected_items, items
     end
   end
+
+  context "#formatted_permission_name" do
+    should "return the permission name if permission is not the signin permission" do
+      assert_equal "Editor", formatted_permission_name("Whitehall", "Editor")
+    end
+
+    should "include the application name if permission is the signin permission" do
+      assert_equal "Has access to Whitehall?", formatted_permission_name("Whitehall", SupportedPermission::SIGNIN_NAME)
+    end
+  end
 end