Update our Mirador viewer JS

[thatbudakguy]

I hastily added some JS during the GBL v4 upgrade so that we would use Mirador instead of Clover as the viewer for IIIF content – this way things would appear more consistent for Earthworks patrons, since anything using sul-embed for IIIF will be using mirador already.

We need to:

• update this JS to become a stimulus controller, modeled after the viewer controllers in geoblacklight
• make sure that it works for both IIIF manifest items and IIIF single-image items (see the clover viewer controller in GBL for how to do this)