Part of [WAF statistics reporting][1].

This improves the integration with the backend for WAF statistics
reporting in a few ways.

In particular, this:

- Handles 404 responses from the WAF statistics endpoint not as errors,
  but as an indication that there is no data to display.
- Displays a new empty state when there is no data to display.
- Handles 204 No Content responses from the WAF statisics endpoint, and
  continues to poll until a 200 OK response (or any error) is received.
  To do this, the existing `pollUntilComplete` utility was moved to
  a more shared location, and tests were added for it.

[1]: https://gitlab.com/gitlab-org/gitlab/issues/14707