Migrate to Helix 3

[rofe]

These are likely the relevant steps to migrate this project to Helix 3:

• Add Helix Bot to repo
• Move any server-side includes/embeds to the client-side (except head.html)
• Change origin to Helix 3 Preview URL in CDN
• Add hlx3 flag to Sidekick config so it uses Helix Admin API