A state parameter was added for the routes that require user authentication. Now, everytime a statechange occurs, the destination state is checked and user is redirected to signin page if necessary. Note the state parameter is added within `data`, so that nested states can inherent its value.
Emails are made unique. When user attempts to sign in through a provider in which his email is one that is already registered, user is redirected to the signin page with an error passed as a query string parameter.
