Make email case normalization configurable

[mike-marcacci]

There is not universal consistency regarding the treatment of address case sensitivity by email providers. This means that some systems would treat NAME@example.com as distinct from name@example.com, while others would treat them as the same.

An inconsistency here in either direction can create security and usability issues (although not of particularly high severity). Regardless, this behavior should be configurable at the authority level.