Improve token access add form

What does this MR do and why?

Improve token access add form

Show form feedback when group/project path is already in allowlist
Replace text box with group/project dropdown

Changelog: changed

MR acceptance checklist

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Screenshots or screen recordings

Before	After
Screen_Recording_2024-06-07_at_11.55.15	Screen_Recording_2024-06-07_at_12.25.45

How to set up and validate locally

1. navigate to a project
2. in the sidebar, choose Settings => CI/CD
3. expand the Job token permissions section (previously known as Token Access)
4. add groups and projects to the project's allowlist
5. the form should:
   • show a list of groups and projects
   • be searchable
   • warn you if you try to add the same group/project twice
   • add the selected group/project to the allowlist on submit
   • be cleared on cancel

Related to #415519