Store AKA hosts in the database

This means we will record hits against AKA hostnames when they appear in the
logs from our CDN.

This has made the `has_aka` field on Host and the code for finding aka hosts in
Transition::Import::DnsDetails confusing, hopefully I can simplify that...
Relate canonical and aka hosts

This allows us to simplify the Host model and the code for fetching DNS
details.