Send a JSON object in POST instead of CGI form data

With this change, the request body sent to the server is actually a proper JSON object instead of a flat list of CGI variables with nested names.

Further encoding the URL and HTML becomes unnecessary as an added bonus.

This is what we're using internally as an experimental integration.  (Parsing JSON is easier for us than de-nesting CGI variables.)

I also looked into cleaning up the Base86 image data prior to sending, but for now I'm removing the extra 22 prefix characters server-side.