Local download shouldn't upload to Imgur

[nfriend]

Currently, downloading a creation involves the following steps:

1. User clicks the "Download image"
2. The image is uploaded to Imgur
3. The user is redirected to a page that embeds the uploaded image from Imgur

We could simplify this a bit by using canvas.toDataUrl():

1. User clicks the "Download image"
2. The application calls canvas.toDataUrl() and downloads the file locally.

The main advantage of this approach is that local downloading would work even if Imgur is blocked, as it is in many context (i.e.schools).