Fix Vite Leaflet CSS import

What does this MR do and why?

Fix Leaflet styles import for Vite production build.

Vite has a problem with resolving relative URL imports in CSS (at least in our configuration). When it encounters an import like this in CSS: url(images/foo.png) it tries to resolve it relative to the assets folder (/app/assets) instead relative to the module folder (node_modules/some_package). A simple fix here would be to explicitly specify this is a relative URL: url(./images/foo.png). We can apply a yarn patch in our case to fix this.