New demos automatically run the latest content snapshots

[chillu]

Acceptance Criteria

• New demos run the latest snapshot available from the "content loading" environment (which contains the "master content")

Notes

• Could use the SCP API: https://platform.silverstripe.com/static/api.html#snapshots-snapshot-get
• Avoids redeploying ss.com which currently contains the snapshot identifier. This is particularly important since there might be unrelated changes waiting to be deployed on ss.com, making it a mult-team effort to push through a one-line change
• Related: New demos run latest stable release