[6.x] Fix authenticating existing users if the underlying users email has been changed

[joelbutcher]

Resolves #350

This PR makes a few changes to the authentication flow & responses:

1. Updates the canRegister method to check $user and $account values. If either of these are NOT null it returns false, instructing us to attempt to authenticate the existing user.
2. If we cannot find a user for the email on the provider, we now resolve the user from the connected account model, if one exists for the provider.
3. Added a new test to ensure users who change their emails can still be logged in with previous providers
4. Deprecated some response contracts and classes in favour of a more generic OAuthFailed event and OAuthFailedResponse.
5. Replaces all usages of session() with Session facade

[joelbutcher]

\cc. @mystyq – this should fix the problems you're having

[mystyq]

@joelbutcher Yes, this fixes resolving the correct account. Thank you.