Avoid email duplication by capitalization

[giobber]

Django normalize_email doesn't force email to be lowercase or point insensitive before @ symbol

For more info see: https://stackoverflow.com/questions/74754013/is-there-an-efficient-way-of-preventing-the-same-email-addresses-registered-with

• [ ] Create a custom normalize_email which convert email to a lowercase version without point or other strange symbol
• [ ] Add a validator to check if email is lowercase

[giobber]

Some additional notes

• GMail permits usage of plus symbol to create on-the-fly alias (example: name.surname+social@gmail.com is equivalent to name.surname@gmail.com but user can know from which site that email come)
• & = _ ‘ - + are the only suggested symbols (see https://mailchimp.com/it/resources/are-email-addresses-case-sensitive/)
• email are case insensitive: we can save the lowercase version and check over that