Support aka.foo.com == www.foo.com aka hostnames

We use a different aka hostname based on whether or not the original hostname began with 'www.'. My basis for this is that we have tests in redirector which try eg http://aka.homeoffice.gov.uk which pass against redirector but fail against bouncer.

Hopefully @annashipman or @norm can tell whether or not I've understood the (intention of the) redirector's behaviour correctly? That's the primary reason for the Pull Request. We can have a chat so I can explain Bouncer if that would help.