Onboarding experiment frontend tracking

The following frontend event is tracked:

• The user dismissed the popover (action popover_dismissed)

Note: this issues depends on !33194 (merged) being merged first. After merging, I will switch the target branch back to master.

Part of issue: gitlab-org/growth/product#107 (closed)

There is no changelog, since all changes from this MR are behind the experiment onboarding_issues, which is controlled by the feature flag onboarding_issues_experiment_percentage.