add support for everyone option to is_active_for_user

[jdoconnor]

fixes #401

This adds support of the everyone boolean to is_active_for_user

[jwhitlock]

Should this also return False if everyone is False, and only continue on a per-user check if everyone is None? This is how flag.is_active works:

https://github.com/django-waffle/django-waffle/blob/63444d6f83a3c75a9630a02e32cfe65ac1305b86/waffle/models.py#L282-L285

[chess-octane]

I agree with @jwhitlock . I wonder what are your thoughts @clintonb ?

[chess-octane]

I submitted a PR to tackle that: #498