Refine email hashing

[maybeanerd]

Currently emails are hashed using a shared salt that's provided by the environment. This is good enough if you trust your provider to not attempt to try guessing emails, but we can probably do better and make this more difficult to reverse.

Research what better ways might exist and if we move, provide on the fly migrations of users using a temporary column "oldEmailHash" or similar

Potentially interesting resource: https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html