Count the no. of PMs, not the number of appointments
Make historic_appointments#show more sensible

We don't actually need a role appointment when displaying a persons
historical account. This only loads the things we are interested in.
Merge pull request #498 from alphagov/previous-pms-count

Count the no. of PMs, not the number of appointments

https://www.pivotaltracker.com/story/show/48719635