[Blocked] "Save & add another" with no values goes to next stage

**This is blocked until https://github.com/okfn/ckan/pull/1117 gets merged so that I can write some tests.**

In the second stage of dataset creation (_Add data_) if you leave all the fields blank and hit _Save & add another_ you get a validation error: _You must add at least one data resource_. All fine.

But if you add one resouece, hit _Save & add another_, then leave all the fields blank and hit _Save & add another_ again, instead of giving you a validation error it just goes straight to stage 3 of dataset creation (and doesn't create a second resource at all).

This also happens if you're uploading a file to the FileStore and hit _Save & add another_ before it's finished uploading.

The best solution is probably for the _Save & add another_ button to be disabled, until some info has been added.