2753: Remove older, unused Workbench provenance code.

I do not see any code that would directly render these partials;
they're not indirectly referenced in the pane list; and I can remove
the controller code and run all our tests without tripping on a
NoMethodError exception from a nil.
2753: Add event_at data to Log fixtures.
2753: Move Workbench object list rendering to application_helper.

I would like to be able to reuse this functionality in the upcoming
Collections view.
2753: Show immediate relations on Workbench Collection page.

This concept and implementation borrows a lot from the new folder UI.
Merge branch '2753-collection-metadata'

Closes #2780, #2837.