Add Roles and Seeds

[npauzenga]

Hey @enriikke I'm going to adjust my handling of GameNight organizers and Group moderators to use Roles instead of attributes.

I'm using a gem here. I would have preferred to implement these associations myself but in the interest of finding gems to contribute to, I'm going to use an off-the-shelf solution.

[enriikke]

I've never used rolify before but it looks great. You've already worked with pundit so I think is good to try something else.

Awesome work! 😄