Rework file upload for #360 and #361

Rework file upload to support large files over slow connections and add progress indicator during upload.

Files are uploaded directly to Windows Azure storage using Shared Access Signature. For developers deploying their own site, they will need to enable CORS on their blob service using the new command provided in the fab file. For a Dev deployment, the deployment config does not require modifications.

The implementation uses the File APIs. Thus it requires a sufficiently modern browser.

There are still more ways in which the implementation could be improved: 1) upload multiple blocks concurrently; 2) retry strategy if a single block upload fails.