Crop thumbnail images as well as scaling them

Images are cropped from the center of the image horizontally and from the
top edge vertically.

Addresses #276
Use CSS to crop story image thumbnails in explorer

Addresses #276.
Disable featured image cropping

For now, try using CSS cropping instead of actually resizing the image.

Addresses #276
Merge branch 'develop' into 520-viewer_asset_border