Add QR code generation to image functions for offline permalinks

[BartG95]

This might sound a bit odd for a website, but the idea is to add a QR code of the permalink of a page to that page itself, possibly only displayed by a @media print css rule. Then when someone encounters the printed version, they can easily go to the up to date online version.

[bep]

Yea, that would be useful.

This looks solid:

https://github.com/rsc/qr

I suggest we make it:

{{ images.QR "https://example.org" }}

Which would produce a image resource that could be used as an overlay.

[jmooring]

Having gone through a similar exercise in a document management context, we will want to test this by printing a page at 600 dpi, then scanning with a phone.