User mapping - Add email templates for reassign request

What does this MR do and why?

Add template and preview for import_source_user_reassign. This MR only includes the template (frontend) and not the backend logic that sends the email.

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

Format	Screenshot
HTML
Text

How to set up and validate locally

1. Optional, only needed if you don't already have any Import::SourceUser records:
   1. Checkout the branch seed-placeholder-users-task.
   2. Run this rake task bin/rake gitlab:seed:placeholder_users\[22,10\], replacing 22 with any group_id. (Note: 10 is the number of users created)
   3. Open a Rails console with bin/rails console and run this:

s = Import::SourceUser.last
s.reassigned_by_user_id = 1
s.reassign_to_user_id = s.placeholder_user_id
s.status = 1
s.save

2. Checkout this branch and restart GDK (⚠️ Important: Restarting is needed to view the email previews as they seem to be cached).
3. Go to http://127.0.0.1:3000/rails/mailers and preview the import_source_user_reassign email template:
   • HTML: http://127.0.0.1:3000/rails/mailers/notify/import_source_user_reassign.html?locale=en
   • Text: http://127.0.0.1:3000/rails/mailers/notify/import_source_user_reassign.txt?locale=en

Related to #455912 (closed)