Replace user_select in fogbugz/new_user_map/index.js

What does this MR do and why?

This MR replaces old legacy select2-based implementation with new shiny component, based on gl-listbox

Screenshots or screen recordings

Screencast_10_17_2022_07_00_31_PM

Previous version (can be seen at https://gitlab.com/import/fogbugz/new with same credentials provided below):

❗ There was no ability to select "no user" once you've selected something in dropdown

New version:

(no user) will always be seen, no matter of filtering, allowing to unassign current user

Out of the scope of MR:

• selecting items in dropdown cause table columns width to change. This is both for old and new version

How to set up and validate locally

• open /import/fogbugz/new
• use credentials:
  • fogbugz url: https://importtest3.fogbugz.com/
  • fogbugz email: iklymov+fogbugz2@gitlab.com
  • password: import
• Observe new dropdown in Gitlab User
• Pick something in one or more dropdowns
• (open debugger, disconnect from network)
• Submit form
• Observe submitted data in devtools

For each user where you selected gitlab_user there should be field users[${id}][gitlab_user] with id of local gitlab user For other users it should be empty

Example:

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 #369993 (closed)