User mapping - Add tab counts for placeholders child tabs

What does this MR do and why?

• Add tab counts for "Awaiting reassignment" / "Reassigned" tabs that are sent from backend via helpers + state.
• Add logic to update the tab count when "Keep as placeholder" action is selected.

MR acceptance checklist

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Screenshots or screen recordings

User_mapping_tab_counts

Before	After

How to set up and validate locally

1. Create a few dummy Import::SourceUser records:
   1. Checkout the branch seed-placeholder-users-task.
   2. Run this rake task bin/rake gitlab:seed:placeholder_users\[22,50\], replacing 22 with any group_id. (Note: 50 is the number of users created)
2. Go to Group > Members and select the Placeholders tab. For example, http://127.0.0.1:3000/groups/gitlab-org/-/group_members?tab=placeholders.

Related to #471932 (closed)