Configurable user agent for IIIF image checks

https://redmine.teklia.com/issues/7700

The requests.get call made by arkindex.images.models.Image.perform_check should set the User-Agent header using a new settings.IIIF_USER_AGENT setting.

This setting is configurable in the YAML configuration, as iiif_user_agent: .... It defaults to Arkindex/{VERSION} (+https://teklia.com/) (VERSION is already available in the settings).

Please update existing unit tests to check for this new header, and add a new test where the IIIF_USER_AGENT is set to something else, to check that the setting does change the user agent.