Update the `MarkdownEditor` component to use `Icon`, `IconButton`,
`LabeledButton` and `Link` where relevant. Reduce complexity of
local component CSS and use utility styles where possible. Extract
button and link style overrides to `frontend_shared` SASS module.

Part of https://github.com/hypothesis/client/issues/3876
Part of https://github.com/hypothesis/frontend-shared/issues/232