Handle resizing nicely

Summary:
  - For images that aren't too large to fit, keep the stage at about 85% of the viewport height. This prevents it from bouncing around as you switch between images, and generally makes it feel big and important like it should. When the stage is larger than images, we center them vertically.
  - As the viewport is resized, shrink the stage and, if necessary, the images on it.

Test Plan:
{F33617}

{F33618}

Reviewers: chad, ljalonen

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5089