Large font/accessibility settings

[adhameer]

We would like to be able to adjust the font size for the exercises and playground (in the editor, toplevel, and description, etc.). We have tried doing this by just zooming in, but after zooming in past a certain point the interface changes to the single-column mode, while we would like to keep it with the editor and description side-by-side.

So we're left with manually adjusting the fonts in learnocaml_common.ml: https://github.com/ocaml-sf/learn-ocaml/blob/718d1f12ff2ef78b1db34d2f70e4b24ddad0ed53/src/app/learnocaml_common.ml#L706 and in various places in the CSS, and then rebuilding the package.

Could the font size be made adjustable?