Convert identifier lookup to list
Convert identifier lookup to list

This is because `@cached_property` doesn't cache callable queryset
methods, meaning that a new query to all PersonIdentifiers was called
each time `get_single_identifier_of_type` was called on the same
instance.

Using and caching a list reduces the number of duplicate queries

DemocracyClub/yournextrepresentative,DemocracyClub/yournextrepresentative,DemocracyClub/yournextrepresentative