Backdrop read api should always return UTC

Consumers of backdrop will always want datetimes to be represented as
UTC. Previously they had no timezone attached, which was rather confusing
as the ISODate in mongo had a timezone.

This commit will force all datetime objects coming out of backdrop into
UTC before writing them out to JSON.
Merge pull request #272 from alphagov/always-utc-out

Backdrop read api should always return UTC