Create a web app manifest

[toddjordan]

Part of making the app a PWA, we need to make a good web-app manifest: https://developer.mozilla.org/en-US/docs/Web/Manifest

[oskarrough]

https://github.com/san650/ember-web-app

[sivakumar-kailasam]

The theme color and logo images to be used can be seen in the styles.

[sivakumar-kailasam]

Addressed by #210