UI clean-ups on cluster health charts

As part of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10245, there are a few UI clean-ups that need to be made as part of a separate issue/MR.

• Adding tick marks to the chart
• Updating the formatting of the popover
• Improving the responsiveness of the popover

Further details are in: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10245#note_152627267 and https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10245#note_152997658