Font size picker

[alexdln]

We've already switched to font sizes entirely in rem units, and spacing entirely in px.

Overall, the interface is already friendly to external font-size-enlarging programs and works correctly, increasing the size as expected.

However, it would also be a nice-to-have option to add a Text-size picker to our website to: control the font-size-enlargement behavior ourselves and provide a11y tool out-of-the-box

Discord thread