Add a link from Differential to Diffusion

Summary:
Provide an easy way to jump to Diffusion from Differential if we have
the data we need to connect them.

Test Plan: Tested menu in linked and unlinked diffs. Used menu item.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley, btrahan

Maniphest Tasks: T309

Differential Revision: https://secure.phabricator.com/D1326