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.
Oh yes, good point, especially given [this](https://github.com/alphagov/transition/pull/445/files#diff-a5d3de5ca0f4ba404e8ac1c5d6f3c8aaR10) test data. Done.