support for email addresses with IPv[46] literals

I am now self-hosted without DNS on IPv6 (freeeedooooom!). I cannot add my new email address with the format mailbox@[ipv6:...] as it is blocked by validation

Start by locating the email-address validation entry point and the existing checks for IPv4 or IPv6 literals. Add coverage for the reported mailbox@[ipv6:...] format and verify that valid email addresses continue to be accepted while malformed addresses remain blocked.

Written by the indexing model from the issue text.