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.
Make spec deterministic in the face of array order
Merge pull request #236 from alphagov/store_aka_hosts_in_database

Store AKA hosts in database/Show AKA hits