Implement bulk of Threat Monitoring store and filters component

What does this MR do?

Part of WAF statistics reporting.

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)

Screenshots

Loaded

Note that this requires a patch to be applied, as the feature is not yet complete:

Before	After

Popover

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers (Will do this in a later MR once the feature is more or less complete)