Enable Mark Filled for Expired Listings

### Is your feature request related to a problem? Please describe
The mark filled feature only applies to active listings, but many jobs expire long before a candidate is hired. So employers can't use the mark filled feature for many of their listings. As a site owner, we need candidates to know when jobs expire because they consistently inquire wanting to know this information.

### Describe the solution you'd like
Enable the mark filled feature for expired listings by way of a setting in WPJM's admin panel.

No files, tests, or entry points are identified in the issue. Trace the admin setting and the existing mark-filled behavior for expired listings, then define completion as allowing site owners to enable marking expired listings as filled and confirming that candidates can see the resulting status.

Written by the indexing model from the issue text.