Bug 1094091 - [Stingray] Scaffolding Apps Deck of Stingray

https://bugzilla.mozilla.org/show_bug.cgi?id=1094091
UX spec does not have definition here because spec for App Deck is not completed yet. The value is derived from 10 vw, which is subject to change once spec is available. But I'll put a constant and comment here.