Change prism theme to dark.
Add cancancan gem for authorisation.
Migration: add role column to users table.
Add role to user model's attr_accessible.
Add method to user model for checking if they have a specific role.
Generate cancan ability model.
Rescue from cancan exception by redirecting to referer or root path, with message.
Add method to user model to check if user is admin.
Define abilities file.

Guests can read anything.
Guests can create persisted users who can create anything or destroy
things that belong to them.
Admins can manage anything.