Resizing: choose unit of measure

[c4lliope]

Some choices: em, rem, vh, vw, vmin, vmax, px, %

Some sample code allowing a choice between px <-> rem is here:
https://github.com/assembleapp/donors/blob/main/app/javascript/packs/board.js#L52-L53;
the sample code assumes the font size is 16px;
perhaps we can make a similar reasonable assumption,
or query the browser's stylesheet API for a more precise value.