Platform: Use component to emit event on Subscription toggle component instead of eventHub

What does this MR do?

This MR adds platform changes to subscription.vue component used in Issue and MR sidebars where it changes the way event is emitted (using component event instead of eventHub). This MR is a dependency for frontend changes required in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5496.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/5480