Show confirmation modal for bulk add-on seat assignment and unassignment

What does this MR do and why?

Show confirmation modal for bulk add-on seat assignment and unassignment

• Create a new component for confirmation modal
• Add tests for changes

This MR is one of multiple MRs that implement bulk add-on assignment functionality.

MR	Status
Add ability select multiple users	!145889 (merged)
Show confirmation modal with selected users	👈 👈 👈 You are here
API integration to assign add-on to selected users	🚧

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.

Scenario	Screenshot
Bulk actions
Bulk add-on assignment confirmation - 1 user
Bulk add-on unassignment confirmation - 1 user
Bulk add-on assignment confirmation - multiple users
Bulk add-on unassignment confirmation - multiple users

How to set up and validate locally

Application setup

1. Customers Dot needs to be setup in order to view the billing page for a group
2. Start GDK in SaaS mode GITLAB_SIMULATE_SAAS=1 gdk start

Group setup

1. Create a Group
2. Buy a Subscription for your group from Settings > Billing

Feature flag

Run the following in your rails console

Feature.enable(:hamilton_seat_management, Namespace.find(your-group-id))
Feature.enable(:gitlab_com_duo_pro_bulk_user_assignment, Namespace.find(your-group-id))

Add-on purchase

Purchase an add-on for the subscription purchased above in Group setup step by following the instructions here (under Add Code Suggestions Add-On to a subscription section).

Code Suggestions tab

1. Visit /groups/<your-namespace-name>/-/usage_quotas#code-suggestions-usage-tab

Related to #442162