Restore the "Download from Git LFS" UI button to Diffusion

Summary: Depends on D18827. Ref T7789. See PHI204. See PHI131. This button got accidentally removed in Diffusion refactoring ($data is no longer used).

Test Plan: {F5321459}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T7789

Differential Revision: https://secure.phabricator.com/D18828