add `datetime` format (note, not the built-in `date-time`) to our json
schemas. this uses the iso8601 library to try and parse the string.
also, move `strict-rfc3339` and `rfc3987` (used by jsonschema to
validate `date-time` and `uri` formats respectively from test
requirements to regular requirements. if they're not installed,
validation silently succeeds, so validation wouldnt reject anything bad
on prod, only in unit tests.