Always scale images to fit in Pholio

Summary:
Ref T2641. Currently, we scale images to fit horizontally, but let them have arbitrary vertical size. This is nice in theory but kind of sucks in practice because it makes everything below the stage jump around when you switch images. It would also make swiping through images on mobile super weird.

Instead, scale to fit in both dimensions. This feels a lot better and more application-like to me. (I also think most mocks are not especially tall?)

Test Plan:
{F34648}

(Note that the image is enormous.)

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2641

Differential Revision: https://secure.phabricator.com/D5233