Summary: Also provide a way to update old files metadata.
Test Plan: Create a revision which includes a image file. Check whether the widht, height metadata exists. Run `scripts/files/manage_files.php metadata --all` to update previously uploaded files.
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T2101
Differential Revision: https://secure.phabricator.com/D4347
