Method that gives us roles with unique people

When displaying roles and their current appointments, we don't want
to list people more than once. This generates an array of presenters
with only the first instance of a person present.
Decorate a persons current roles
Simplify test
Wire up controller and views
Improve test coverage

We don't want unappointed roles present
RolesPresenter class for dealing with collections

This wraps the concept of a collection of roles into its own class.
This allows us to grab the unique people and multiple roles that a
given person may have within the collection
Remove unused methods
Remove unnecessary attr_reader statement
Memoize calculation
Don't clobber unique_people
Memoize where it makes sense to
Merge pull request #251 from alphagov/dedup-management-teams

Dedup management teams

https://www.pivotaltracker.com/story/show/44701821
https://www.pivotaltracker.com/story/show/44654127