User mapping - FE - Add modal for CSV download

What does this MR do and why?

This MR is part of #455906 and adds the basic CSV reassignment modal and the ability to download a pre-filled CSV template

Here is the list of MRs around the feature:

MR	Status
CSV reassignment modal + CSV download	👈 You are here
Upload reviewed CSV file	🚧 WIP: work in progress

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

	Screenshot
CSV Button
Modal

How to set up and validate locally

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

Related to #455906