Issue-1572: Support multiple openseadragon viewer windows

[seth-shaw-unlv]

GitHub Issue: Islandora/documentation#1572

What does this Pull Request do?

Adds viewer ID key to openseadragon JS settings allowing us to have multiple open seadragon viewers on the same page.

What's new?

• Changes the attached drupal settings for openseadragon from a single object to a dictionary keyed on the open seadragon viewer window the settings will apply too.
• Changes the openseadragon_viewer.js so that it iterates over the viewer window id => settings pair to enable the viewers.
• Does this change require documentation to be updated? No.
• Does this change add any new dependencies? No.
• Does this change require any other modifications to be made to the repository
  (i.e. Regeneration activity, etc.)? Just clearing cache.

How should this be tested?

• Create a collection of several items.
• Update the members view (http://localhost:8000/admin/structure/views/view/members?destination=/admin/structure/views) to use the 'Open Seadragon' view mode instead of 'Teaser'.
• Clear cache.
• View the collection. Only one member object will have the open seadragon viewer working; the others will have big black rectangles.
• Apply the PR.
• Clear cache.
• View the collection again. All the objects will display their own open seadragon viewer.

Interested parties

@Islandora/8-x-committers

[jordandukart]

Smoke tested this and it seems sound to me, causes the entries in drupalSettings to be keyed uniquely which allows n viewers to be embeded in a page where as previously values would be overwritten.

[manez]

Another successful test here. Works as advertised.

[dannylamb]

I consider this RTBC and am happy to merge.

[dannylamb]

Thanks for testing @jordandukart and @manez