Scrolling epics UX enhancements/bugs

Overview

Follow-up on Roadmap enhancement done within https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9018 as part of https://gitlab.com/gitlab-org/gitlab-ee/issues/7325.

Enhancements

• Prevent flashing of existing epics while timeline is being scrolled and new epics get added.
  • This can be done by prevent re-render of existing epics until new epics are fetched and then do single render at once.
• Use skeleton loading instead of spinner for initial load.
  • Additionally, we can render timeline (without Epics) as soon as page loads as timeline is computed on frontend and once epics are fetched, we render those.