Add 'app install' button for mobile users

When mobile users access GeriLife, we would like to make it easy for them to add a GeriLife shortcut to their home screen.

## Task
Add an 'app install' prompt to GeriLife. The prompt should be minimally invasive, or perhaps accessed by a menu item.

## Resources
See the following for an example of the desired functionality:
- https://developers.google.com/web/fundamentals/app-install-banners/