Add capabilities to user accounts

[JohnRDOrazio]

Taking into consideration that there will necessarily be a few users that give assistance for the final website / service, those users should have "admin" accounts such that they can view the list of "user resources" (basically the list of users that have signed up to the website).
This means adding "capability" information to user accounts (I avoid using the term "role" here, because we've already used the term "role" in reference to the role of a user within a project, as "bride", "groom", "celebrant" or "guest" in the project). Users that sign up to the website will automatically receive a "subscriber" capability, whereas the few of us that manage the service will have a "manager" capability. Only users with a "manager" capability should be able to access the "Users" resource (http://localhost:3000/en/users) with a list of users currently signed up to the website.

[JohnRDOrazio]

I believe this gem should assist in this: https://github.com/CanCanCommunity/cancancan