Create custom validator for json form date-time widget to allow partial dates

[janette]

https://github.com/rjsf-team/react-jsonschema-form

The built in date-time widget forces the user to submit a full date-time value, we need a widget that will make it easy to add dates to the "Issued" field but also allow for simple dates such as 2019 or 2015-03

[janette]

closing in favor of #312