Alx/fea/be/login w username

[alx-sch]

• Restrict allowed characters for usernames (no whitespace, emojis, or special characters) to prevent breaking the username-slug endpoint @dovy-mus is working on.
• The original name the user chooses is now saved in a new displayName column, while the name column stores a normalized, all-lowercase version.
• Users can now log in using their username, and the input is case-insensitive (alice, Alice, and aLiCe all work).

[alx-sch]

the displayName (e.g. aLiCe)(as provided by the user during registration) should show everywhere BUT in the admin dashboard (it wasn't readily available in the schemas there --- but that's quite alright to have the normalized usernames there, I guess (e.g. alice).

[dovy-mus]

I made a new migration that should backfill existing users' names and displayNames in prod -> hopefully works. Also there were a tiny issue with generateNanoid used for the google oauth users it was called 2 times resulting in different displayName and name - now it should be alright