This is somewhat distinct from "zoom level". Users with any vision irritation could use larger text without needing to reach for the full zoom to make everything bigger. Providing a user-level setting for this would help broaden the app's utility for users that might not be served by our default font-size choices. Could use a "multiplier" number field so that users don't have to think in terms of pixels, rems, or anything like that.

Spun out of #8076.