Site-wide temporary lock

[s3ththompson]

When executing a custom deploy script for new versions of my site, I'd like to lock all panel edits at the beginning of the deploy script and unlock when the deploy is successful.

This would be very helpful for ensuring zero-downtime deployments and making sure that edits during the deploy process don't clobber or break anything happening with the deploy script.

(Specifically, this is an issue I'm running into while using the kirby-git-content plugin.)