Part of [WAF statistics reporting][1].

This adds a WAF statistics summary component and a chart for viewing the
history of WAF traffic.

In addition, this:

- Updates the expected response structure from the WAF statistics
  endpoint, according to the latest from the ongoing [backend MR][2].
- Removes the link from the popover per [this discussion][3].
- Stops pushing the `threat_monitoring` feature flag to the frontend,
  since it wasn't being read anyway, and the controller itself is
  guarded behind the flag, which is sufficient.

[1]: https://gitlab.com/gitlab-org/gitlab/issues/14707
[2]: https://gitlab.com/gitlab-org/gitlab/merge_requests/19789
[3]: https://gitlab.com/gitlab-org/gitlab/issues/14707/designs/ee_14707-waf-statistics-reporting_popover.png#note_260158615