Add Threat Monitoring skeleton page

What does this MR do?

Part of WAF statistics reporting, this adds:

• A skeleton page and sidebar nav link for WAF statistics reporting, with access control equivalent to the Project Security Dashboard
• threat_monitoring GitLab Ultimate feature
• threat_monitoring feature flag (disabled by default)
• Additional unit tests for the EE projects helper

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• 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