Replace user_select in app/assets/javascripts/pages/admin/groups/show/index.js

We're in the process of migrating away all uses of Bootstrap. Whilst this is a simple task for things such as buttons and alerts, dropdowns are much more complex. Because of that, there will be no step-by-step instructions in this issue.

This dropdown can (very likely) be migrated to our user_select Vue component, for more info see the epic: &8531

File: app/assets/javascripts/pages/admin/groups/show/index.js

Implementation plan

• Mount a new Vue component in app/views/admin/groups/show.html.haml#L104
• This Vue component will use app/assets/javascripts/invite_members/components/members_token_select.vue
• Use GlListbox to render the access levels dropdown

Availability and Testing

• If members_token_select.vue component will be updated, please run package-and-qa to ensure no regressions occur, as this dropdown is used in other areas of the system. Otherwise, package-and-qa can be skipped, since no E2E tests target the users or access levels dropdown on the app/views/admin/groups/show.html.haml page at this time.