Add image selection for item embed viewers

[taylor-steve]

Closes #2604.

Continuation & partial refactor of the work done in #2752.
Still needs sul-dlss/sul-embed#2279 to work (additional changes to come).

This can be run locally. For example if sul-embed is running at localhost:3001:

# config/initializers/oembed.rb
purl_provider = OEmbed::Provider.new('http://localhost:3001/embed.json?&hide_title=true')

projectblacklight/spotlight#3431 causes a problem when there are multiple copies of the same item in a single embed block. In that case, the panels have duplicate IDs and the first panel will always get the image selection update regardless of which panel the area selection was initiated from. Will need to fix that first.

Changes from #2752:

• Isolates widget modification & viewer state sync to their own classes
• Removes the mutation observer in favor of specific events
• Reduces the scope of the SirTrevor overrides
• Removes the wildcard origin in postMessage
• Syncs the state between the existing Spotlight "multi-image selector" page selection and the viewer