Refactor asset sizing handling (#807).

Fairly major refactoring of how we handle JS-powered asset sizing
in viewer and builder.

- Move towards unifying resizing logic
- Figure containers collapse to small images
- Large images expand to figure container responsively
- Iframe-sandboxed asset behavior unchanged
- Note: re-added imagesLoaded jQuery plugin
Merge branch '807-imagesLoaded' of https://github.com/patternleaf/atlas

This merges in a refactor that addresses #807.

Conflicts:
	static/js/viewer/views.js
Update changelog for #807