web: codemod `web/nav` 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.

Steps to product this PR

1. Prepare codemod package locally:

git clone https://github.com/sourcegraph/codemod 
cd codemod && yarn

2. Run codemod over React component in the nav folder:

yarn transform -t globalCssToCssModule --write true "/sourcegraph/client/web/src/nav/**/*.tsx"

3. Go through Post codemod steps in the codemod README.md

Changes

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