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.
What I was trying to say was that it would be easier - the code in Redirector isn't easily extensible at the moment. 

The other thing pulling me in this direction is the thought that we, or someone else will get rid of a separate repo for configuration and do it in-app, at which point we'd need the validations in-app.