Enhancement: Provide a public PURL Service web form

In order to support and encourage universal PURL adoption and improved standards, please consider a new web form, open to the general public, that supports at least 3 things when you enter (or paste) a PURL into an input field:

* Validate the PURL and provide meaningful, helpful feedback if there are problems with it
* Construct a Download URL, when possible, and present that in a copy-enabled, click-enabled field
* Look for the PURL in a public instance of purldb and present that metadata, or a link to a public display of that metadata

Also find ways to publicize the availability of this web form.
Also explain how it works and how you can use available tools and API's to do the same thing yourself.