Allow same username with diffrent case variations if they are not taken

[mikozet]

If my username was "miko" and I wanted to edit to "Miko", I should be able to do it as long as the username with other case variations is not taken by other account. Right now it says that the username is taken, but actually it's not used by other account, so it should be allowed (unless ther's other user with username "MIKO" or similar)

Currently await core.utils.requestAPI({ path: ['users'], method: 'POST', data: { username, }, }); returns 409