Greatly reduce the number of queries done by OrganisationDetailView

By adding the select_related('person', 'title') the number of queries is
reduced from thousands to 7.
ZA: Correct the party percentages on organisation pages

These calculations previously wrongly included positions in the
organisation over all time - this commit changes this to only consider
people currently in the organisation.

Fixes the first part of #1450.
Remove an unnecessary save after create