Don't clobber @military_roles in OrganisationsController

Use @military_personnel instead.
Allow RolesPresenter to reduce the collection to filled roles only
Make all roles/people fetched in OrganisationsController unique

Follow the pattern established by board_members of using a RolesPresenter and retrieving unique_people from it.  For some reason only the ministers were being reduced to the filled roles so I've replicated that behaviour, but I think that iterating over the unique people has the same effect on the others.
Merge pull request #292 from alphagov/de-dupe-all-the-organisation-roles

De dupe all the organisation roles

https://www.pivotaltracker.com/story/show/45252461