Add tokens for the various different icon sizes

[bawjensen]

On iOS I was doing some work to implement a shimmer loading view for a cell that contained 3 labels and an icon view. We have useful tokens to be able to keep the height of the labels accurate to their actual counterparts (in sass it'd be $tp-font__body__2__line-height), however the icons have no corresponding tokens.

@danoc pointed out that the SVGs don't actually depend on tokens since they have their height/width embedded, or hard-coded. It's possible that having 2 sources of truth might introduce issues.