Feature flags from GitLab

Summary

Integrate GitLab onto feature flags management

• Closes #966 (closed).
• Closes #1298 (closed).

Steps to test

• Go to https://gitlab.com/minds/engine/-/feature_flags
• Click the pencil icon on the media-modal row.
• Notice the [review-site] environment state (flipped on=true, off=false)
• Open https://epic-feature-flags.minds.io/api/v1/minds/config in Postman (or Firefox)
• Navigate to features > media-modal. Check the value matches what you saw on GitLab.
• Now switch it.
• Wait 5 minutes or more (there's a small cache)
• Reload https://epic-feature-flags.minds.io/api/v1/minds/config in Postman (or Firefox)
• Navigate to features > media-modal. Check the value reflects your switch on GitLab.

Note: Ensure you sync with other testers because you might be flipping each other switches.

Estimated Regression Scope

Feature flags might not be correctly read or exported to frontend.

Deployment

New Settings

$CONFIG->set('unleash', [
    'apiUrl' => '', // URL to GitLab's Unleash server
    'instanceId' => '', // GitLab's Unleash instance ID
    'applicationName' => '', // GitLab Feature Flags environment
    'pollingIntervalSeconds' => 300, // Cache bust
    'metricsIntervalSeconds' => 15, // Metrics interval
]);