A11Y: set modal widths with EMs; improves scaling (PR #14863)

Sets various widths on modals to em units instead of pixels. This allows the modals to scale their width along with a user-defined font-size. Previously a large user font-size could cause modal elements to overflow because the px width didn’t scale. I also added a viewport-based max-width in some situations, to prevent overflow on narrow screens with extra large font-sizes set.

Before (html font-size set to 20px):