Replace GitHub project import GlDropdown with GlListbox

What does this MR do and why?

• Dropdown is now fluid width (up to the maximum allowed by Pajamas).
• Once selected, dropdown toggle text is limited to 24 characters so groups that have long names don't break the page.
• "Users" list group is now shown before "Group" list group as the user group will always only have 1 element (thus makes it easier to see).

Changelog: changed

Screenshots or screen recordings

Before	After
GitHub project import dropdown before.mov	GitHub project import dropdown after.mov

How to set up and validate locally

GitHub import (docs)

1. In the header bar, click on + icon (Create new...) > New project / repository.
2. Select Import project > GitHub.
3. Fill in a Person Access Token (with repo scope) and click Authenticate.
4. You should be redirected to the GitHub import status page. For example, http://127.0.0.1:3000/import/github/status.

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 #321190 (closed)