Problem

What an image inside of an Image Block is focused, it isn't clear what is focused. We should add an outline of sorts to provide some visual emphasis.

Note: this highlight should only be displayed when the image itself is tapped to focus. It should be removed when the user taps outside of the block, on another block, or on the Image Block's Caption.

Spec

Either using box-shadow or border (inset):

• 2dp/2pt stroke (inset, as to not upset the alignment)
• $blue-medium aka #00aadc
• Semi-transparent (40% alpha) overlay of black

Note: This should also apply to images inside of Gallery Blocks when we eventually support them.