Add property for alternative domains

Some providers have multiple domains. That makes it possible for users to choose the domain they like. But having an account on a domain that is not the main domain might confuse users when they need to contact the provider's support addresses or visit the provider's website. If each alternative domain would have an own entry in the provider list (i.e., on the same level as the providers' main domains), a listing also displayed some providers having many domains disproportionately. Thus, we decided to not add entries for alternative domains to the provider list.

But we would like to add the alternative domains to the list as additional data in some way. By adding a list for alternative domains to each provider entry, clients are free to suggest those domains to their users. If we do that, we maybe should also add the alternative domains to the list used for autocomplete. But that list is currently the simple list (list containing only domains) of category D providers. Therefore, we probably should extract an additional list from the provider list containing all main and alternative domains. That means, the resulting list would be a simple list of category D providers including their alternative domains.