Building Ionic2 app for browser

[robinyo]

I use:

npm run build --prod

See:

• https://robferguson.org/blog/2018/04/16/0ptimising-the-performance-of-an-ionic-pwa-part-1/
• https://robferguson.org/blog/2018/04/17/optimising-the-performance-of-an-ionic-pwa-part-2/