Add option to Stats cards to remove max-width style rule

Currently every Stats card option (small, medium, large and Extra large) has its own maximum width set along side with other properties. 

https://design.undp.org/?path=/story/components-ui-components-cards-stats-card--stats-card-story

We need to introduce an option to add `.full-width` class which will reset that property and let stats card occupy entire container
