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.
It depends on your definition of the system boundary. Transition can't be configured without redirector (more properly transition-config). That repo *is* part of transition the system (can't function without it, despite it being in a physically "other" place), and doing this here makes for some belt-and-braces noise we can avoid. That said, I don't feel particularly strongly about it. I suppose we already validate host uniqueness over there *and* over here, and consistency can win the day.