Begin laying frontend framework for Threat Monitoring UI

What does this MR do?

Begins laying framework for the Threat Monitoring UI.

Part of WAF statistics reporting.

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.

Screenshots

Empty state

Non-empty state

Note: This state cannot yet be reached without applying (something like) the following patch: fake_waf_setup.patch

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 instead in the MR which enables the feature flag by default.)