Created by: pvdlg
Fixes #20617 (closed)
Generate btn-{breakpoint}-block classes so the block level button style can be applied for each viewport below a given breakpoint.
Handling the vertical spacing in a responsible would have generated a lot of css (see comment.
So I opted to get rid of the margin-top for consecutive btn-block and instead mentionned in the docs to use the spacing utilities.
It seems also more consistent with the changes in the alpha 6 that now relied more on the spacing utilities.