global: backport of Flask-IIIF extension
- NOTE Backports Flask-IIIF extension from original commit 213b6f1144734c9ecf425a1bc7b78e56ee5e4e3e. The extension is not enabled by default in order to avoid feature addition to existing minor release.
Signed-off-by: Øystein Blixhavn <oystein@blixhavn.no>