Improve UX on targeted messages delete

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After moving targeted messages delete to a background job: the following discussion from !214170 should be addressed:

• @bmarjanovic started a discussion: (+2 comments)

  question: What about target message that is already scheduled for deletion?

  Ideally, we should add pending_delete or marked_for_deletion_on attributes on the targeted_messages table. Then, we can wrap actions together and update the attributes. With those, we can improve UX with some flag, and remove/disable delete button 💡