Add placeholder icon and update image card and collection card placeholder state

[fswlee]

At present, when there is no image available for an image card or a collection card, we show a grey background with an information icon and a message "Image preview not available". See image card and collection card.

We want to make several updates:

1. add the placeholder image as an icon
2. the state of the card should be called something like a 'placeholder' state rather than an error state
3. we want to use a placeholder image to better communicate the issue at hand for the user--that there is no image at present.

3a. the placeholder image should be vertically and horizontally aligned within the square
3b. the small version of the image should be used for the image card (48 width) and the large for the collection card (64 width). I've supplied versions in both sizes

[daneah]

@fswlee does it make sense to add the placeholder as part of the system iconography instead?

[fswlee]

sorry i missed this @daneah -- I didn't see the notification email. I think that would make sense. That seems like the best place for the placeholder image to sit within the system