Add site upload to snippets

[mtanco]

I can never remember how to get an image "into" wave to use elsewhere, would love to see this as a snippet:

(q.app.header_image,) = await q.site.upload(["static/overview.png"])

[mturoci]

Good suggestion, tracked in: #1239

[azim-b]

Also, more importantly, use this "new" path, returned by q.site.upload, as the image path to serve the image.