Add individual dropdown

What does this MR do and why?

Describe in detail what your merge request does and why.

Replace user/group selection with add individual dropdown.

• behind a feature flag
• group selection coming in subsequent MR/s (see issue for breakdown)
• remove select2 component for a dropdown
• create user_select
• allow for adding users in a project
• populate dropdown with existing users

Screenshots or screen recordings

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

NOTE: The issues with the dropdown text box will be addressed in a separate issues (FE: Fix search-box-by-type input box in Gl-Form (#381649 - closed) and FE: Display selected users/groups (#382148 - closed)) due to their complexity and this MR being the first of several.

Type	Screenshot	Screenshot	Screenshot	Screenshot
Before Screenshots
After Screenshots

How to set up and validate locally

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

1. Upload a GitLab Ultimate license
2. Enable the scan_result_role_action feature flag: echo "Feature.enable(:scan_result_role_action)" | rails c
3. Navigate to http://gdk.test:3443/flightjs/Flight/-/security/policies/new?type=scan_result_policy
4. Verify users can add users

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #377864 (closed)