Usernames instead of emails as identities

[ferferga]

Describe the feature

In the context of selfhosting, the email address doesn't make much sense for user identities. Emails imply that they're used for delivering notifications (which currently it's impossible, although I guess that could be configured in the future with a mail server in case it's necessary) and unique (which may not be the case for multiple self-hosted instances).

If, in the future

Why is this feature important?

• We're not forced to any "conventions" for logging in to our instances or naming users.
• Multiple accounts are not as "awakward": for my use case, I want to have an account in a household with my partner, and another account in another household for my family. Being forced to use emails make me more difficult to remember which account is which.
• Email addresses as identities are not as relevant in the context of self hosted software (as proven by the identity schemas that other self-hosted software follows).
• It's confusing for newbies in the selfhosting world, since a verification code is usually expected after using your email addresses while signing up.

Additional context, screenshots, and relevant links

Even for the future manager instance provided by Maybe this is a good addition, since some people prefer logging in with their short nickname instead of full email.