Add v3 image helper

[glenrobson]

For an item which has a single image its possible to do:

https://iiif.archive.org/iiif/img-8664_202009/full/512,/0/default.jpg

but you can't add a version so:

https://iiif.archive.org/iiif/3/img-8664_202009/full/512,/0/default.jpg

doesn't work.

iiif/iiify/app.py

Line 287 in 43fa078

@app.route('/iiif/<identifier>/<region>/<size>/<rotation>/<quality>.<fmt>')

v2 version used in the IIIF training:

https://training.iiif.io/iiif-online-workshop/day-two/image-servers/iiif-hosting-ia.html