Optimize JS bundle size when doing production builds

When we ship the JS built by webpack, we should ship a 'production' version that's optimized for size, etc. Otherwise the bundle sizes seem pretty big:

```
➜ npm run webpack

> nbgitpuller@0.10.1 webpack
> webpack

asset bundle.js 415 KiB [compared for emit] [minimized] [big] (name: main) 2 related assets
runtime modules 663 bytes 3 modules
orphan modules 1.04 KiB [orphan] 1 module
modules by path ../../node_modules/ 608 KiB
modules by path ../../node_modules/style-loader/dist/runtime/*.js 5.02 KiB 6 modules
modules by path ../../node_modules/xterm/ 315 KiB
../../node_modules/xterm/lib/xterm.js 306 KiB [built] [code generated]
../../node_modules/css-loader/dist/cjs.js!../../node_modules/xterm/css/xterm.css 9.81 KiB [built] [code generated]
modules by path ../../node_modules/css-loader/dist/runtime/*.js 3.85 KiB
../../node_modules/css-loader/dist/runtime/cssWithMappingToString.js 2.28 KiB [built] [code generated]
../../node_modules/css-loader/dist/runtime/api.js 1.57 KiB [built] [code generated]
../../node_modules/xterm-addon-fit/lib/xterm-addon-fit.js 1.68 KiB [built] [code generated]
../../node_modules/jquery/dist/jquery.js 282 KiB [built] [code generated]
./js/index.js + 1 modules 10 KiB [built] [code generated]

WARNING in configuration
The 'mode' option has not been set, webpack will fallback to 'production' for this value.
Set 'mode' option to 'development' or 'production' to enable defaults for each environment.
You can also set it to 'none' to disable any default behavior. Learn more: https://webpack.js.org/configuration/mode/

WARNING in asset size limit: The following asset(s) exceed the recommended size limit (244 KiB).
This can impact web performance.
Assets:
bundle.js (415 KiB)

WARNING in entrypoint size limit: The following entrypoint(s) combined asset size exceeds the recommended limit (244 KiB). This can impact web performance.
Entrypoints:
main (415 KiB)
bundle.js

WARNING in webpack performance recommendations:
You can limit the size of your bundles by using import() or require.ensure to lazy load some parts of your application.
For more info visit https://webpack.js.org/guides/code-splitting/

```

_Originally posted by @yuvipanda in https://github.com/jupyterhub/nbgitpuller/issues/197#issuecomment-902820238_