Lowercase hosts on import and when adding to the whitelist

- Lowercase hosts when importing from site YAML files, so we don't potentially add uppercase hosts that Bouncer can't handle.
- Lowercase hosts when they're added to the whitelist.
This test passes even if the code it is testing is reverted. That's because `www.attorney-general.gov.uk` gets imported on this line: https://github.com/alphagov/transition/blob/lowercase_hosts_in_import_and_whitelist/spec/lib/transition/import/site_yaml_file_spec.rb#L7

I think the solution would be to choose a different alias.
 
Additionally, `site.hosts.pluck(:aliases)` needs to be `site.hosts.pluck(:hostname)`.