make enketo subdomain generation and redirection work with non-default ports.
substituted HTTP_HOST with SERVER_NAME where used because the former can be manipulated by the client
could be cleaned up more to minimise code duplication, I wanted to get it to work first and I have to delve a little deeper into CIs structure to do that. but we could probably do more with the the subdomain helper in the survey model?
Merge pull request #320 from scornflakes/implement_custom_ports_for_subdomains

make subdomain generation and redirection work with non-default ports