Lowercase hosts on import and reject uppercase hosts for the whitelist

- Lowercase hosts when importing from site YAML files, so we don't potentially add uppercase hosts that Bouncer can't handle.
- Explicitly reject adding an uppercase host to the whitelist.
Did we say in sprint planning we were going to fail the redirector build if values like this got in? That way if anyone that wasn't us did enter in future, for example, `My.New.Host.gov.uk`, it would not make it past CI and hence couldn't be used as a right-looking example by any subsequent naive editors.