Fix #140: Add support for offline development via auto-login backend.

I think we need to be clear here that to disable autologin, they need to remove **both** things. Because the helper is **only** looking at `BROWSERID_AUTOLOGIN_EMAIL` when it figures out whether to add the shim or not.

Either that or I'm misreading that code.

I'm being pedantic here because this is a huge shoot-yourself-in-the-foot kind of thing. I'd prefer if we were checking for both things instead of just `BROWSERID_AUTOLOGIN_EMAIL` since that seems less likely to cause unintentional calamity.