Add last_modified and created to settings and workspace API items

[bollwyvl]

• fixes Expose more file metadata about disk-backed content #98

Here's a minimal proposal for adding the fields mentioned in #98 to the various API routes for workspaces and settings.

• A workspace may or may not have metadata.created or .last_modified, but if it appears, should always be an UTC ISO 8601 datetime.
  • since it gets PUT back, the user could modify the value, and it would be written to disk, but then overwritten when read back out. seems a little odd, but still okay
• A settings item will always have both created and last_modified, and can be a UTC ISO 8601 datetime, or null in the event that the user hasn't created a settings file (otherwise, there was actually no way to know, as it always returns at least {} in raw).
  • this value is never written to disk

These both use the newly shimmed tz in server, but as this doesn't provide a way to parse UTC ISO strings, the tests now also use what jsonschema would use, strict-rfc3339... and indeed, it, and a few other packages, as runtime dependencies might provide better conformance to the JSON Schema spec.

Longer term... these handlers would probably benefit from some schema, which would go part of the way towards jupyterlab/jupyterlab#6013.

[bollwyvl]

haha, no fromisoformat for me, py3.5/3.6!

[blink1073]

LGTM, thanks!