Versioning

[graft]

When a new verison of an existing file is uploaded, instead of deleting the old file it should be kept as a previous version.

1. The old binary is moved to .
2. The new binary is created in place at
3. A 'Version' record is created for the file, tracking the old binary's md5sum and the backup_id for the old version, if any.
4. The file's json record reports a list of previous versions, if any, as a list of md5sum.
5. The file download API responds to a version parameter which downloads the given md5sum revision of the indicated file.
6. The UI allows the user to examine the list of versions for a file and download one of them.

[graft]

File revision quiz

Files A and B are in folder (C). File D is in folder (E). What is the version history of file A under the following circumstances?

1. A new file is uploaded with name A.
2. File A is deleted. A new file is uploaded with name A.
3. File A is renamed to F.
4. File A is copied to F.
5. File A is copied to folder (E).
6. File A is renamed to B (B is replaced).
7. File A is moved over file D (D is replaced).
8. Folder (C) is renamed to (G).