Use thumbnail and posterCanvas from IIIF Manifest

[joncameron]

Description

Poster and thumbnail support will be added to the iiif_manifest gem and will need to then be parsed and displayed by the Avalon player.

Done Looks Like

• The player uses the posterCanvas image from within a IIIF manifest as the poster image
• The player retains a reference to the thumbnail image (use within the player itself is currently undefined)

Note

Blocked by samvera-labs/avalon-bundle#275 and samvera-labs/avalon-bundle#277