Add an aka_hostname method to Host

We don't want or need to store the aka hostnames as separate hosts
for a site, because we don't need to use them within the app. They
are needed for configuring the CDN, but we can generate them for
that purpose from the real hostnames.
Include aka hostnames in the hosts API

We need to configure the CDN to handle these hostnames in the same
way as their real hostnames, so they should be included in the data
we expose for that purpose.
Add response info and total into hosts API response

This is the minimum needed to make the response format consistent with
Whitehall's APIs, not including pagination which we don't need at the
moment.
Always return a JSON response for hosts index
Test hosts presenter in hosts presenter tests

The main logic for generating the response was moved from the
hosts controller into the hosts presenter, so the tests for it
should also be moved.