Add a script to migrate files between storage engines

Summary: Quora requested this (moving to S3) but it's also clearly a good idea.

Test Plan:
Ran with various valid/invalid options to test options. Error/sanity checking seemed OK.

Migrated individual local files.

Migrated all my local files back and forth between engines several times.

Uploaded some new files.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1950

Differential Revision: https://secure.phabricator.com/D3808