Bold track information on the flag when hovering on a specific track

As part of https://gitlab.com/gitlab-org/gitlab-ce/issues/41783 I encountered some difficulties when implementing this, as it made the movement over the graphs choppy.

  • Bolded line item when user hovers over a specific line:

graph-popover--key-highlighted

cc @tauriedavis and @joshlambert as part of our zoom call

Edited by silv