Do not return empty fields from APIv2 for users

Eventually, this change will be made to apply to all API endpoints, but it
is useful for debugging while the API is developed.
Return BSON datetimes as ISO 8601 when converting to JSON
Add basic support for APIv2 PATCH's "remove" operation
Merge branch 'apiv2_updates'

* apiv2_updates:
  Add basic support for APIv2 PATCH's "remove" operation
  Return BSON datetimes as ISO 8601 when converting to JSON
  Do not return empty fields from APIv2 for users