Add `o-container__pad` element class

[spaceninja]

In https://github.com/cloudfour/cloudfour.com-wp/issues/780 Tyler noted:

One possible solution would be to add a o-container__pad element class to the pattern library to compliment the existing o-container__fill and o-container__fill-pad element classes. This could then be added to the Deck block's class attribute when certain alignment options are selected.