Ensured no duplicate people on ministerial list

I've used Tekin's RolesPresenter which contains helpers to ensure we
have a unique group of people. This class is really a domain class
rather than forming part of the presentation layer: it's a good
representation of a sub-group of roles for an organisation.

This has radically simplified the `ministers_by_organisation` method in
the `MinisterialRolesController` and removed the need for the select
workaround.
Ensure no duplicate people shown in whips section

Propagate the changes made in the previous commit to the whips section,
using the RolesPresenter to show the person information.
Merge pull request #312 from alphagov/remove-duplicate-people-entries

Remove duplicate people entries

https://www.pivotaltracker.com/story/show/45673943