Add basic working version of backup/restore
Add fs-extra package
Add GrainInfo struct to grain.capnp and persist it through backup.

Backup also includes log now, although it isn't used in restore.
GrainInfo is encoded and is named metadata in the archive. The sandbox's
data now all goes into a folder called data.
Change restore to be a global function. Also add icon for download
Various fixups to backup/restore
Add recent files to gitignore