Date format validation for CMA and AAIB reports

Custom validator checks dates are parsable by Ruby Date library and of
form YYYY-MM-DD.
Merge pull request #152 from alphagov/validate-dates

Date format validation for CMA and AAIB reports
Redirect /specialist-documents to /cma-cases

Permanently redirect old urls to specialist-documents to /cma-case
Merge pull request #150 from alphagov/redirect-specialist-documents-to-cma-cases

Redirect /specialist-documents to /cma-cases
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.