Make standalone url optional

This seems to work as expected. One thing I noticed is that the description text for the setting requires the entity type to exist. When creating the new entity, the machine name does not yet exist so it is not populated.

I would also like to see someone test upgrading from the existing behavior to ensure that existing URLs for entities are maintained after the upgrade and that disabling URLs on those entities works as expected.

I removed the 'at
' portion of the field description. You're raising a good point about the upgrade path, we should probably add an update hook that sets the value for existing entity types. Setting back to Needs work.

I added an update hook that sets the standalone_url option to true for existing entity types, to make sure existing behaviour is preserved.