Port Labels Select dropdown to Vue

What does this MR do?

Ports labels select dropdown to Vue with full functionality

Why was this MR needed?

Currently labels dropdown is rendered partly via HAML for markup and labels_select.js for core functionality. This MR makes Vue component that can be used in place of HAML to render labels dropdown in any Vue app/component. Since core markup remains the same, there's no visual change in the dropdown, it is only behind the scenes where Vue is used to manipulate the dropdown.

This MR is also a foundation for EE feature https://gitlab.com/gitlab-org/gitlab-ee/issues/3727.

Screenshots

Default View	Create Label	Selected Labels

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests added for this feature/bug
• Review
  • [ ] Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/3727

https://gitlab.com/gitlab-org/gitlab-ee/issues/4032