[Feature] Allow configuration of login redirect route via config

[robertmarney]

Closes #1704

Description:

Allow developers to override the opinion of where unauthenticated users are redirected to for login.

This is particularly useful in cases where the application has multiple guards in place.

Implementation:

1. Introduce new config property to store the custom login route.
2. Introduce new make function on the existing Laravel\Passport\Exceptions\AuthenticationException and use the existing redirectTo argument.
3. Replace all existing throw new AuthenticationException with throw AuthenticationException::make()
4. Update existing tests with the requisite mocks
5. Introduce new test for this explicit functionality

[taylorotwell]

Catch the exception in your application's exception handler's render method and return the redirect from there.