Draft: Attempt the union approach on emoji association on epic and work item

What does this MR do and why?

re #443543 (closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

1. make sure to have epic_creation_with_synced_work_item FF enabled
2. Create a couple epics in a give Group G
3. Open rails console
4. Try the following queries:
   1. Epic.last.award_emoji
   2. Epic.last.work_item.award_emoji
   3. EpicsFinder.new(current_user, {my_reaction_emoji: 'thumbsup', group_id: 24}).execute
   4. WorkItems::WorkItemsFinder.new(current_user, {my_reaction_emoji:'thumbsup', group_id: 24}).execute