display thumbnail in public collection page. Fixes issue #3358

[blancoj]

Fixes #3358

If an image has been selected to serve as the thumbnail for a collection, then the thumbnail should be visible in the descriptive metadata area of the public show page for the collection. Before this PR, the representative image was not showing up. I have placed the image right above the "Collection Details". It seemed to me that if the collection had a banner and a long title this is where it would look the best.

Guidance for testing, such as acceptance criteria or new user interface behaviors:

• Create a public collection
• Create a public work that includes an image file
• Put the work in the collection
• Edit the collection and select the work's image as the thumbnail for the collection
• Navigate to the collection's public show page. Notice that the representative thumbnail is right above the Collection Details.

You can experiment by adding a banner to the collection and title and see how it looks. Attached is an image of how it rendered for me:

@samvera/hyrax-code-reviewers

[elrayle]

Looks good

[elrayle]

Still looks good