shared.css: Move sup, sub styling from commonElements to shared styles

This really should be an even more basic style than I thought when
moving this CSS here from Vector-specific stylesheet.

Follow-up to Ie98f8b57 and Ic5ba8363.

Bug: 49965
Change-Id: I80f8279e10277d1a7389c821037d733ccfc4e166
Merge "shared.css: Move sup, sub styling from commonElements to shared styles"