Scroll vertically in roadmap view

Follow up to https://gitlab.com/gitlab-org/gitlab-ee/issues/7325, to see if we need to do any clean up for vertical scrolling.

In particular:

When scrolling vertically, additional epics are loaded from the backend to the frontend. They should be then displayed. Since they have been loaded to the frontend, even if you scroll back up and then back down later, they are already available in the frontend and so are rendered immediately.

Since the newly loaded epics should follow the existing sort order of the list, any newly loaded epics would appear at the bottom.

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.