Catch case where older election IDs were causing a 404
Catch case where older election IDs were causing a 404

The JSON for the diff stores the ID as it was at the time it was created.

This was causing 404 errors when `get_by_slug` was called on it

neavouli/yournextrepresentative,DemocracyClub/yournextrepresentative,mysociety/yournextmp-popit,neavouli/yournextrepresentative,DemocracyClub/yournextrepresentative,neavouli/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextmp-popit,mysociety/yournextmp-popit,mysociety/yournextmp-popit,neavouli/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextrepresentative,neavouli/yournextrepresentative,mysociety/yournextrepresentative,DemocracyClub/yournextrepresentative,mysociety/yournextmp-popit