Make Place.related_people include positions, and take a position filter

The people who are returned by Place.related_people were determined by
their positions, but those positions weren't easily accessible in the
template that displays them; this was confusing because you wouldn't
see *why* those people were being displayed.

In addition, each country has different criteria for considering
positions important (e.g. in some countries it might only be MPs,
in some it might be senators, and anyone who's a member of a particular
house).  To deal with this, related_people now takes a position filter
which defaults to a country-specific function.
Also use related_people for child places

It was a strange inconsistency that different criteria were used
in a uniform list of people when finding those associated
with the place itself, and those associated with a child place.
This commit switches to using related_people for both.
Factor out common template code in the place's people page
If there are no positions for child places, don't output the heading

This adds a helper method called related_people_child_places, which
returns the results of related_people for each child place, and
uses that to return the list of people associated with child places.
Make the Place overview page only show relevant representatives

There was a strange inconsistency where the People sub-page for
a Place showed:

  - people who had a current significant position associated with
    the place

  - and people who had a current significant position associated
    with any child place of the place

... whereas the overview page would show people who had any
political position associated with the place (i.e. it would
include extra people who had positions in a particular council
for a place, or local party heads for the place, etc.)  As well
as the inconsistency, it wouldn't be immediately clear why those
people were appearing on the overview page, and they weren't
really "representing" the page as the section heading suggested.

This commit changes the overview page to also use the
related_people method to find those to show on the front page.