fix UserCredential filters in django admin

[deborahgu]

the various sidebar filters in django admin no longer are working.

Steps to reproduce

1. Go to: https://your_credentials_url/admin/credentials/usercredential/
2. Click "Awarded" under "Filter By status" in the sidebar.
3. Note that all revoked certs are still in there.

Acceptance criteria

1. all of the various sidebar filters actually filter the page results
2. the page doesn't load noticeably slower than it previously did