Dashboard partner allow not only staff

This changes the partner dashboard user management such that

1) all users can be associated with a partner (previously only is_staff=True)
2) when a user is associated with a partner, he gets the partner:dashboard_access permission (previously he got is_staff=True)
3) same when a new user is created through the partner dashboard
4) when an existing user is unlinked from the partner the partner:dashboard_access permission is removed from the user.

I'm not sure how associated users are really supposed to work, but as I understand the code, the association makes sense only if the user has access to the dashboard. Also, if the user has is_staff=True, the various filters that allow only partner-specific orders and products to be accessed are circumvented. Therefore I concluded that the idea is for users associated with a partner to have be allowed access to the dashboard with is_staff=False and the partner:dashboard_access permission.