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?
@czue @kkrampa We shouldn't use a signal for this - I think for now we can just add another handler under settings.SMS_HANDLERS to accomplish this, and make sure that the handler returns False for any non-ilsgateway domains so that it will be skipped.  Longer term we may support having a domain be able to plug-in custom handlers so we might end up moving it out of SMS_HANDLERS and into a config option, but for now I think it's ok to just add to SMS_HANDLERS.

For an example sms handler, see corehq.apps.sms.handlers.form_session.form_session_handler for the function signature. It should return True if the message was processed (so that no other handlers will get it), and False if it was not processed (so that other handlers can pick it up).