Have a better message look-and-feel when a user is already confirmed

[epicfaace]

Going to https://root.treehacks.com/verify?username=ccdc5059-4a20-4782-aa2e-705cc0375d27&code=842690

Gives me

We should probably redirect to the login page so the failure is silent (because who cares if the user is already confirmed -- they just want to log in!)