Convert `addon-knobs` to `addon-controls`

[ScopeyNZ]

Storybooks' addon-knobs has been deprecated in favour of addon-controls: https://storybook.js.org/addons/@storybook/addon-knobs

addon-controls looks like it's a better suite for this feature, so it would be nice to switch over - especially before there's a new minor version of Storybook to update to.