Fix sign in color for successful sign-in actions

See https://github.com/codeforamerica/bizfriendly-web/issues/152

I don't have valid credentials to test this, but it should work - like other actions already in BFuser.js, whenever the alert-success class is added to the message box, jQuery should also remove the alert-danger class - because once added, it'll override alert-success.

P.S. apologies for the commits with dev environment files - I think they're useful to have, but I'm not good enough with the git to figure out how to cherry pick commits yet.