add support for external image proxying

[tripodsan]

This issue is about implementing the external image support explained in adobe/helix-pages#93 for details.

Suggestion

• helix-static receives a path that matches /${base64_encoded}.external.image
• helix-static detects the external image request and decodes the url
• rejects requests with missing or invalid accept headers (404 or 415)
• helix-static fetches the external content.
• if the content-length of the response is < MAX_BYTES the content is returned accordingly.
• otherwise the sha1 of the content is computed
• it checks if the respective blob is already available
• otherwise it uploads it to azure
• sends a 307 redirect to the azure blob location.

notes

• downloading can happen to memory until a certain threshold is reached but then would go to disk
• if the external url is already a helix blob store url, don't refetch the content

[trieloff]

downloading can happen to memory until a certain threshold is reached but then would go to disk

If this is still interesting, I think with request we can just pipe the download and upload, so that we never have to hit the disk and keep the memory footprint quite small.

[tripodsan]

I recently discussed this with @davidnuescheler and we probably should not upload external images to our blob store. doing this transparently might cause copyright problems.