Suggested edits to further conform to ISO 8601 temporal standards

It looks like this has been brought up before in (https://github.com/project-open-data/project-open-data.github.io/pull/43) but a solid commit was not made to align fully with the ISO 8601 standard. I suggest expressing datetimes as `YYYY-MM-DDTHH:MM:SSZ` and ranges as `YYYY-MM-DDTHH:MM:SSZ/YYYY-MM-DDTHH:MM:SSZ`.