Remove `date` attribute from default blog post frontmatter
When omitted, it's inferred from the filename. It's only required when setting a specific publication time, so we've added notes to the README.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
When omitted, it's inferred from the filename. It's only required when setting a specific publication time, so we've added notes to the README.