Preload images in Pholio and fix dimension rendering

Summary:
Preload all the images when viewing a pholio. Fixes T2692.

We should probably use lower-resolution previews on mobile and only show full resolution when the user taps. We should also suppress the thumbnails from loading on mobile since they aren't visible. But neither is critical for the moment.

Test Plan: Added logging, verified everything loaded.

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2692

Differential Revision: https://secure.phabricator.com/D5316