Remember username after failed login attempt

When my login attempts fail, 99% of the time it's because I miss-typed the password. After miss-typing the password, I have to re-type both username and password, which is annoying.

This commit stores the supplied username upon a failed login attempt, and when showing the login-form again, the previous username is supplied in the login-field. The password field then gets keyboard-focus.

This way, only the password has to be re-entered when it was miss-typed, making the login process much easier.