Add Created column to /admin/reports/rl experiments overview

[jjroelofs]

Summary

The experiments overview at /admin/reports/rl currently shows columns for Operations, Experiment, Source, Impressions, Conversions, Variants, and Last Activity. It does not surface when an experiment was first registered.

Proposal

Add a Created column populated from er.registered_at (already selected by ExperimentDataStorage::getExperimentsWithStats()), formatted via DateFormatter::format($ts, 'short'). Place it next to Last Activity so the two timestamps can be compared at a glance.

Why

• Helps operators sort/scan experiments by age.
• Distinguishes brand-new experiments (no traffic yet, Last Activity = Never) from old ones that have simply gone quiet.
• Data is already loaded; no new queries required.

Acceptance

• New "Created" column appears in ReportsController::experimentsOverview().
• Renders the formatted registered_at timestamp, or "Never" if zero.