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.common.js

@@ -352,7 +352,7 @@ module.exports = {
 
   output: {
     path: path.join(__dirname, 'public/build'),
-    publicPath: '/build/',
+    publicPath: 'build/',
     filename: '[name].js'
   },