Make Diffusion do an alright job on Git LFS objects

Summary: Ref T7789. This isn't the most perfect UI imaginable, but it's similar to what GitHub does and seems reasonable.

Test Plan:
{F1180271}

{F1180272}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7789

Differential Revision: https://secure.phabricator.com/D15494