Add a `bin/files purge` workflow

Summary:
We can lose file data through various means; one reasonable way is if files get deleted from disk with 'local-disk' storage. If data goes missing,

Ref T3265. Also, reduce some code duplication.

Test Plan:
Ran `bin/files purge`, `bin/files migrate`, `bin/files rebuild` with various args.

Deleted a file with "local-disk" storage, ran `bin/files purge`, made sure it got picked up.

Reviewers: dctrwatson, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3265

Differential Revision: https://secure.phabricator.com/D6068