Fix urlPath support, let CodiMD be served from a subpath correctly

Webpack now uses relative paths for resources linked from by static
snippets. A templated <base> tag has been introduced in headers
so app.js can set the base URL at runtime.

Signed-off-by: Enrico Guiraud <enrico.guiraud@cern.ch>

webpack.prod.js

@@ -9,7 +9,7 @@ module.exports = [
     mode: 'production',
     output: {
       path: path.join(__dirname, 'public/build'),
-      publicPath: '/build/',
+      publicPath: 'build/',
       filename: '[name].[contenthash].js'
     }
   }),