Set webpack-dev-server host to 0.0.0.0 (#1122)

webpack-dev-server CLI has a default value of localhost, which only
allows accessing locally. Setting to 0.0.0.0 allows to access the server
from the external. However, the open option will open 0.0.0.0:7936,
which is an invalid address, so it has been removed.

webpack.dev.js

@@ -13,6 +13,7 @@ katexConfig.devServer = {
     // testing.  This potentially reveals the source code to the world,
     // but this should not be a concern for testing open-source software.
     disableHostCheck: true,
+    host: '0.0.0.0',
     port: PORT,
     stats: {
         colors: true,