This simplifies commit 17a3b5b6b0
The top or bottom margins of contained elements will always collapse together
(per http://seifi.org/css/understanding-taming-collapsing-margins-in-css.html)
What we actually want is a margin bottom in tablet mode for Special:MobileOptions
and a padding bottom for taglines that only exists if the tagline is not empty
Bug: T202557
Change-Id: I741f08d2dec04f94b7c233a04a3fca7a065520f8