Email Uniqueness: Support IDNs and improve uniqueness handling

[HeyJoel]

• Email uniqueness checks aren't enforced in the domain, only usernames. This causes an issue if emails aren't used for usernames
• Email constraints are not case-sensitive, but they should be for the local part of the email. The domain should not be case sensitive but should try and support IDNs.
• It would be good to add a service for formatting email / domains for uniqueness checks e.g. test@googlemail.com and t.est+123@gmail.com are the same address. The base rules should probably be minimal to avoid false-positives, but should be able to be upgraded via extension/override. Note that uniquification should not be applied to the email address used for notifications, as the original format should be honored.

[HeyJoel]

Fixed, will be released in v0.10. Guidance on customizing the formatting of emails and usernames for uniqueness checks will be added to the docs.