User mapping - FE - Add CSV upload functionality

What does this MR do and why?

This MR is part of #455906 and adds the CSV upload functionality to the modal:

Here is the list of MRs around the feature:

MR	Status
CSV reassignment modal + CSV download	✅ merged
Introduce FF for CSV reassignment	✅ merged
Add CSV reassignment path	✅ merged
Upload reviewed CSV file	👈 You are here

Add CSV upload modal for user contribution reassignments

Changelog: added

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enable(:importer_user_mapping_reassignment_csv)

2. Navigate to a group's members settings, like http://localhost:3000/groups/gitlab-org/-/group_members
3. Select the Placeholders tab.
4. Click Reassign with CSV file button

Related to #455906