Roadmap infinite scrolling UX enhancements

What does this MR do?

This MR is an extension of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9018. It makes following enhancement to Roadmap;

• Use skeleton loading for Roadmap load.
• Prevent timeline bar refresh of existing Epics until new epics are fetched on scroll.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7325

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer

Closes #9245 (closed)