Return display string in compact resource respresentations

[mark-cooper]

Not all records have (or require) titles. However the PUI breadcrumbs for search were expecting a title, which when absent results in an error.

This is resolved by including the display string in compact results and using that as a fallback when title is not present.

Before (ao has no no title, display string not included):

      "_resolved_ancestors": {
        <!-- others ... -->
        "/repositories/2/archival_objects/2684": [
          {
            "publish": true,
            "level": "other level",
            "uri": "/repositories/2/archival_objects/2684"
          }
        ]
      }

After (ao has no title, includes display_string):

      "_resolved_ancestors": {
        <!-- others ... -->
        "/repositories/2/archival_objects/2684": [
          {
            "publish": true,
            "level": "other level",
            "uri": "/repositories/2/archival_objects/2684",
            "display_string": "1909, 1919, undated"
          }
        ]
      }

Steps to reproduce the issue

Create published records: resource -> child 1 (no title) -> child 2 (title)
In PUI search for child 2 title. Will error on master branch.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have authority to submit this code.
• I have added tests to cover my changes.
• All new and existing tests passed.