Import sites from backups

Assuming we #1372612: Keep a copy of makefiles when creating platforms and #1372614: Include platform makefile with site backups, it should be feasible to import sites from backups in a fairly straight-forward manner. We could find the makefile in the backup (platform.make?) and build a new platform based on it. By deploying the site backup to the new platform, we could presumably then just import the site as usual.