Rename bin/files metadata to bin/files rebuild and let it rebuild MIME information

Summary:
See https://github.com/facebook/phabricator/issues/320. Files can end up with a bad MIME type, and we don't update it when uploading another copy of the file since obviously the new copy has the same data and thus the same MIME type.

• Rename bin/files metadata to bin/files rebuild to make it a more consistent verb.
• Let it rebuild MIME types so users who hit issues like this can run bin/files rebuild --all --rebuild-mime to straighten things out.

Test Plan: Ran bin/files in various modes, examined output.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5951