disable publishing button if required fields are not filled out

The easy thing to do would be to add the required attribute to the 'Items to Publish' checkbox. Since it is not supported in Safari, the code is checking to see if all of the required fields are checked or filled out and then enabling the Publish button if true.

I think it would be a good idea to add some type of indicator in the publishing modal for the fields that are required.

Fixes https://github.com/Connexions/webview/issues/552