create a IIIF discovery endpoint for IIIF Manifests

[saracarl]

<moved from https://github.com/ArchiveLabs/iiif.archivelab.org/issues/85>

IIIF has a standard for sharing IIIF content called the Change Discovery API:

https://iiif.io/api/discovery/1.0/

It would be interesting to add this to the IA IIIF endpoint. This could be in addition to the current collection and search functionality we discussed previously.