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