Fix how we handle acknowledging logins

* state is now separated into two categories: login in progress, or no login in progress
* if a login form is detected, then login has failed
* if no login form is detected and there is no check selector, we wait 500ms and then mark the login as successful

This prevents errors where the Waltz credential form was popping up at inappropriate times,
since the page was being checked before a check selector was loaded.