[#56763208] Add a `role` column to the users table.

- Allowed roles are `user` and `super_admin`
[#56763208] Add cancan.
[#56763208] Add a statistics page.
[#56763208] Only super admins can see the admin page.
[#56463014] Add an admin section for welcome pages.
[#56463014] Hook up the `Welcome Pages` link on the statistics page.
[#56463014] Only a super admin can change welcome pages.