Android badge and smartbanner

Adds a badge and smartbanner for the Android app that link to the play store app.

The badge is a small icon in the footer which always appears. We can move it somewhere else though if needed.

The smartbanner appears at the top when you visit only from an Android browser. When dismissed, it won't appear for 30 days. If you follow the link, it won't appear for 90 days. Both of these are configurable.