fix(GlHeatMap): disable pointer events on legend

mentioned in commit e7215389

mentioned in merge request rpadovani/daintree!147 (merged)

mentioned in merge request gitlab!34303 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!1965 (merged)

mentioned in merge request status-page!87 (closed)

mentioned in merge request rpadovani/daintree!151 (merged)

mentioned in merge request status-page!94 (merged)