Make (optionally?) authentication case-insensitive

[ldidry]

Is your feature request related to a problem? Please describe.
People can forget they register with a capital letter (like Username), try to login with username and fail.

Describe the solution you'd like
It would be useful to make the authentication case-insensitive (for the username, not the password, of course).

Additional context
I know that the case sensitivity comes from Django authentication system (😒).

The case-sensitivity could be a setting in order to avoid to break authentication on existing instances.