Make 4 fake tna_timestamps the right length

When these were imported with 2 extra digits, they were silently
coerced to '0000-00-00 00:00:00' in the database, which is then
treated as nil by Rails, which caused errors on their site dashboards
in Transition.
Merge pull request #806 from alphagov/make_timestamps_right_length

Make 4 fake tna_timestamps the right length