Add play store promo

[teolemon]

What

• Add play store promo
• https://developers.google.com/web/updates/2015/03/increasing-engagement-with-app-install-banners-in-chrome-for-android#wzxhzdk9native_app_install_bannerwzxhzdk10

Part of

• Road to mobile app

[hangy]

• Probably fixed with #610?

[teolemon]

The criteria are similar to the Web App install banner except for the need of a Service Worker:

• You have a web app manifest file The manifest defines how your app appears on the user's system and how it should be launched - and you are required to have a short_name and a 144x144 png icon
• Your icon declarations should include a mime type of image/png
• Your site is served over HTTPS
• The user has visited your site twice over two separate days during the course of two weeks. It is simple to integrate into any manifest. Just add a related_applications array with the platforms of play (for Google Play) and the App Id, and then add "prefer_related_applications": true to always show the native app banner before the Web App Install banner.

  "prefer_related_applications": true,
  "related_applications": [
  {
  "platform": "play",
  "id": "com.google.samples.apps.iosched"
  }
  ],

[hangy]

I think the correct icon size might be missing, but most of the icons is missing from Git completely, so I couldn't generate more sizes. I left prefer_related_applications out intentionally. AFAIK, the play store hint should still be displayed on a 2nd visit. We can think about setting it to true with the new app, but I'm not so sure about the old one. :)

[teolemon]

@hangy Let's be more aggressive and put prefer_related_applications >> we now migrated to the new app

[teolemon]

cc @stephanegigandet