Rename policies responsible translation key
Add new policies responsible string when no person

The existing translation key would result in "under the responsibility
of No one is assigned to this role as {%role}" when a ministerial role
didn't have a person in it.
Merge pull request #933 from alphagov/unoccupied-role-policy-text

https://www.pivotaltracker.com/story/show/58420382

Unoccupied role policy text