This commit updates empty state images and copy on the Threat
Monitoring page. Documentation links are also updated to point to the
corresponding sections.

This commit converts existing WAF related components into presentation
components so they could be used for the network policy section

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

This commit removes { sync: false } accross entire codebase
This is useless since upgrade to @vue/test-utils 1.0.0.beta-30

This ensures that once a user dismisses the info alert on the Threat
Monitoring page, it doesn't appear again on subsequent visits.

Part of [WAF statistics reporting][1].

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

localVue is required only with use method

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

Ensure we are updating templates when we're emitting things

Part of [WAF statistics reporting][1].

This adds the bulk of the Vuex store for fetching environments and
the WAF statistics, and the filter bar component which allows selection
of the current environment.

This also:

- Aligns the UI with the latest designs, including the help link
  popover, copy, and the alert variant
- Fixes/improves existing tests (avoiding verbose snapshots)

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

Part of [WAF statistics reporting][1].

This sets up some of the frontend framework for the Threat Monitoring
page, including a basic store and the root app component.

As this feature is not yet complete, it is disabled by default behind
the `threat_monitoring` feature flag.

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