users
----------
user_id      auto-incrementing, primary key
name         text, for human consumtion

entities
----------
entity_id    auto-incrementing, primary key
name         text, for human consumtion
permission   boolean, default permission

groups
----------
group_id     auto-incrementing, primary key
name         text, for human consumtion
priority     int, optional. to give one group priority over another

group/entity membership
----------
group_id     int, references groups table
entity_id    int, references entities table
permission   boolean, this group's permission on the entity

user/entity membership
----------
user_id      int, references users table
entity_id    int, references entities table
permission   boolean, this user's permission on the entity

user/group membership
----------
user_id      int, references users table
entity_id    int, references entities table