[Feature]: Prevent signing up with a username that matches another case-insensitively

[itsalaidbacklife]

Feature Summary

myUsername and myusername should not both be allowed; we should only accept one version of a username and ignore case

Detailed Description

We had an issue where a player accidentally signed up with two different capitalizations of the same username without realizing it. We should prevent this kind of issue by updating api/UserController's signup method to perform a case-insensitive search for users with that username and error if one is found to prevent users from having case-insensitive duplicate usernames