FIX: allow search filtering in ModelAdmin

[Cambis]

Description

• The existing functionality passed an ArrayList to a GridField. During search filtering the search context would throw an error because it expects a DataList instead. To address this I have refactored the existing functionality to use a DataList, new code has been included as methods in order to reduce cognitive complexity.

Manual testing steps

• Go to the workflow model admin
• Confirm that search filtering works for pending and submitted items

Issues

Pull request checklist

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