Mock http requests made using fetch

We currently rely on sinonJS to mock responses on our tests, but it doesn't support the new fetch API, which means we need to force the fetch API to use the polyfill

<script>
   window.fetch = null;
</script>

Tests where we force the polyfill:

• nuxeo-connection
• nuxeo-document
• nuxeo-operation
• nuxeo-resource
• nuxeo-document-distribution-chart
• tests in dataviz repo

Possible solutions:
https://www.npmjs.com/package/fetch-mock