Store width and height metadata of image files

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