Create migration to turn dates into strings.

This migration will turn the opened_date and closed_date on all
cma_cases, including both the bare attributes and extra_fields.

There's a big comment in the code about some weirdness with mongoid,
too.

Thanks mongoid. Thongoid.
Merge pull request #154 from alphagov/normalize_dates_for_specialist_documents

Create migration to turn dates into strings.