Improve deploy to be more fault tolerant, allow rollback.

This will require a restructuring of how we store project code on servers.

Basically there would be 2 (or 3) copies of a project on a "production" server at any given time.

There are three relevant locations (none of these names are final):

• ~/projectroot (hold the git clone)
• ~/live (holds an rsync copy of the project root from the last "deploy")
• /var/www (symlink to ~/live)

When you run an "update", you git pull to update ~/projectroot, but that's not "live" so nothing changes yet. Then you rsync the entire folder into a new ~/staging folder, run composer install and whatever else there, and if that all succeeds, move the symlink to point to it.

Then you have a "rollback" target still in the old ~/live folder that's only a symlink (and maybe migrations run down) away.