Fixes date modified formatting and adds guards for nil values

[sdellis]

Fixes #5525 ; refs #5490

This PR formats the dates properly, provides a "date_added" fallback when "date_modified" is nil, and guards against nil values in the _list_works.html.erb view.

An alternate approach was suggested which involves creating a new solar field, "last_change" (or something like that) which would provide the date_modified or date_added if the resource was not modified.

The preferred approach can be debated on this PR in the comments.

@samvera/hyrax-code-reviewers

[stale]

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