We add a new `use_responsive_design` helper, for pages to opt into this
new CSS—mostly just because like… it's *worse* to apply these styles
for pages that don't expect it 😅
And then, I fix up a couple things on the item page (including in the
general items layout) to match!
I'm doing this because the species face picker layout is going to want
some responsive awareness, and I want to be doing that from the start!
