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, into strings.

There's a big comment in the code about some weirdness with mongoid,
too.

Thanks mongoid. Thongoid.
Why wouldn't this need fixing if it's `nil`? What if it was set to a date once and had since been put back to a `nil`? Wouldn't that suffer from the same issue if we tried to put a date-ish string into in the future?