Support case-insensitive matching of email addresses

Email addresses are very commonly case-insensitive (it is up to their host mail server, but "case-insensitive" is widely implemented, and I believe would nowadays represent the overwhelming majority). Users are therefore used to the case not mattering, and so if they register an account using one version of their email address (perhaps copy/pasting a mixed-case version), and later try to log in by manually typing their address in lower-case, it can cause confusion.

Mail Login should therefore support case-insensitive matching of email addresses as an option when logging in.

If there are multiple case-insensitive matches (multiple registered users with conflicting email addresses when case is ignored) then case-sensitivity should be enforced to ensure the correct user is being authenticated; but if there is only a single match then we can reasonably attempt to authenticate that account.