web: codemod `web/diff` components to CSS modules

Created by: valerybugakov

Context

This PR results from the global-css-to-css-modules codemod application with some manual fixes, caused by the usage of global CSS classes tied to a React component in other parts of the codebase: e.g., .sidebar class used outside of the <Sidebar /> component.

Changes

• The stylesheets in client/web/src/components/diff are converted to CSS modules.