Update scripts and docs to use yarn where appropriate

Most `npm` operations are replaced with `yarn`, which generally has better
behavior. However, steps like publish that write to the NPM registry are left to
`npm`, which currently handles these tasks best.

scripts/electron-package.sh

@@ -85,8 +85,8 @@ if [ -n "$conffile" ]; then
     pushd "$builddir"
 fi
 
-npm install
-npm run build:electron
+yarn install
+yarn build:electron
 
 popd