Add support for case-insensitive usernames

[shayneczyzewski]

Is your feature request related to a problem? Please describe.
Currently, the username field for auth is just a String that is used as-is, and thus, is case-sensitive.

Describe the solution you'd like
It would be nice for UX if we had some support for making them case-insensitive so that "shayne" and "Shayne" would be considered the same username.

Describe alternatives you've considered
We could make this a new flag on the auth dictionary, or maybe also provide some Prisma middleware to handle it so they could do even more elaborate things to normalize.

[sodic]

I think the best approach is starting with a middleware for "mapping" usernames.

Case insensitivity doesn't seem like a property that's intrinsic enough to earn an extra special field inside the language (e.g., as opposed to something like stripping numbers or underscores).

If it proves popular enough and ends up making over 90% of users' middlewares, then sure, we can add it in.

[infomiho]

Since #1583 we have standardised the way we save auth provider user IDs: we lowercase them always. That's what's also recommended by OWASP anyways: https://owasp.deteact.com/cheat/cheatsheets/Authentication_Cheat_Sheet.html#user-ids