Improve deploy to be more fault tolerant, allow rollback.

@ricog y travaille déjà.

Depuis le 15/6/2016.

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.