Ilsgateway integration

@czue I moved changes from ilsgateway-products-integration to this branch and I added webusers integration. I will be pushing all integrations with ilsgateway in this branch.  Is it ok? I assume that you don't want to merge this changes until it will be completely done?
yeah exactly. and i am proposing going one step further and not even making the property, but just using a classmethod on `ILSGatewayConfig` as I mentioned above.

My reasoning is basically that it's not good for any core code to depend on any code in `custom.anything`. We may break this in the current codebase, but we should not be introducing it wherever there is a reasonable alternative