Workflow enhancements

This builds upon my other PR (#10), adding a pair of enhancements that we have found useful

The first is splitting up the available locales into 2 groups: already_translated and not_yet_translated. I use optgroups to display them cleanly.
The second is the addition of the datetime field type (our translations each have their own `published_at` field, which this drives)

Please let me know if you have any questions.

-Ben