Add a "--copy" flag to "bin/files migrate"

Summary:
Ref T11596. When exporting data from the Phacility cluster, we bin/files migrate data from S3 into a database dump on the aux tier.

With current semantics, this moves the data and destroys it in S3.

Add a --copy flag to copy the data instead. This leaves the old copy around, which is what we want for exports.

Test Plan:

• Ran bin/files migrate to go from blob to disk with --copy. Verified a copy was left in the database.
• Copied it back, verified a copy was left on disk (total: 2 database copies, 1 disk copy).
• Moved it back without copy, verified database was destroyed and disk was created (total: 1 database copy, 2 disk copies).
• Moved it back without copy, verified local disk was destroyed and blob was created (total: 2 datbabase copies, 1 disk copy).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11596

Differential Revision: https://secure.phabricator.com/D16497