Adds superuser status

- Method :superuser? added to User class
- Method :superuser_group added to GroupService
- DulHydra.superuser_group config setting added -- defaults to env var value
- Ability class overrides hydra_default_permissions to grant :manage, :all if current_user is superuser
- DulHydra.extra_ability_logic config setting removed -- using Ability#custom_permissions instead, as Hydra::Ability intends.