Use full ISO 8601 date strings

[dempseyatgithub]

Use full ISO 8601 date strings for review period start and end dates.

For the following reasons:

• The current date strings do not work with the .iso8601 date decoding strategy used by JSONDecoder
• Use a single date format throughout JSON file, anticipating adding a timestamp when the JSON file was generated

The dates generated for review periods will match how the SE Dashboard currently interprets the date values, midnight in UTC.

This change will require no change in the SE Dashboard. Its JavaScript will accept the new date string format without changes.