Convert all timestamp fields to Time vs DateTime

[floehopper]

Some timestamp fields (ManualRecord::Edition#originally_published_at & SectionEdition#exported_at) are of type DateTime; whereas timestamp fields defined via Mongoid::Timestamps are of type Time. In my experience, it's better to use Time in Ruby unless you are dealing with different types of calendar, e.g. Gregorian, Julian, etc. If nothing else we should make all timestamp fields have the same type.