Use CSS to resize the lightboxed image instead of Javascript

Summary: We can let the browser do the scaling with some simpler CSS rules.

Test Plan: Opened very large images in Safari, Firefox and Chrome and resized the browser. Observed smooth scaling and no issues with the image overlapping UI elements, etc.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1896

Differential Revision: https://secure.phabricator.com/D3802