Prevent signing up with a username that matches another case-insensitively

[Mang0codes]

#1234

• Resolves #1234

Please check the following

• [ ] Do the tests still pass? (see Run the Tests)
• [x] Is the code formatted properly? (see Linting (Formatting))
• For New Features:
  • [ ] Have tests been added to cover any new features or fixes?
  • [ ] Has the documentation been updated accordingly?

Please describe additional details for testing this change

Now the api/UserController's signup method: normalizes the incoming username to lowercase and performs a case-insensitive search to make usernames case-insensitive and prevent duplicates like MyName vs myname