Use the same icon for "Resigned" in Differential and Diffusion

Summary:
See PHI2015. Currently, "resigned" reviewers/auditors get different icons in Differential and Diffusion.

The Diffusion icon is exceptionally poor and confusing: it does not communicate "resigned" and it is similar to other icons.

For clarity and consistency, use the Differential icon (a grey "X") in both applications.

Test Plan:

Differential Revision: https://secure.phabricator.com/D21593