Fix revert_person_from_version_data's handling of biscuits :)
Fix revert_person_from_version_data's handling of biscuits :)

The person.favourite_biscuit field is serialized to
extra_fields.favourite_biscuits (note the plural, for consistency with
old version data), but revert_person_from_version_data wasn't setting
person.favourite_biscuit back from the version data. This commit fixes
that, so reverting someone to an earlier version also reverts the data
about their favourite biscuits.

Fixes #940 🍪

DemocracyClub/yournextrepresentative,DemocracyClub/yournextrepresentative,DemocracyClub/yournextrepresentative