Integrate GitLab Duo Pro bulk add-on unassignment API with the UI

What does this MR do and why?

Integrate GitLab Duo Pro bulk add-on unassignment API with the UI

• Adds loading state for the API call
• Resets user selection upon successful API call
• Adds tests

All changes are behind a feature flag, no changelog required.

MR	Status
Add ability select multiple users	!145889 (merged)
Show confirmation modal with selected users	!146444 (merged)
API integration to assign add-on to selected users	!147505 (merged)
API integration to un-assign add-on to selected users	👈 👈 👈 You are here
Success alert and error handling	🚧

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
SaaS - Bulk unassignment	Screen_Recording_2024-03-25_at_3.31.13_PM
Loading state - simulated on a slow network	Screen_Recording_2024-03-25_at_3.32.40_PM
API request and response
SM - no change

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 #442372