sidebar: logged in: add 'users' link

This link (and the interface as it looks now) is intended to be used by
admins to create/edit existing users.
role: add seed data

This is actually very useful for deployment because it defines standard
roles that will always be there.
user/role: define one-to-many association

Each User belongs to a single Role. E.g., 20 users will all have the
"author" description from the Role table. And, a single user can only
have 1 role.