Allow getting former related politicians for a place

This makes it easier to split up the lists on the place detail page.
Change Place model to allow retrieving former politicians

This makes it easier to split the lists of people on the place detail
page for SA into current and former members, while still being able to
do a count for the total on the aggregate set of all politicians.
Add former politicians and member counts to place detail context

We'll use this in the place detail template to render up to 5 different
lists and up to 3 people counts.
Split place detail people into former and current members

As requested in #780