Rework landing screen to be child-centric

[rockfordlhotka]

The current landing screen (4 boxes) is really a placeholder.

A better user experience would be for the user to land directly on the list of children, with the other advanced/extra features available from a hamburger menu.

[bseebacher]

Current thinking (Summer 2019) is to rework the UI to use the new Xamarin Forms 4 Shell.

https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/shell/introduction

This new framework does not include built-in support for login workflows. See these links possible workarounds.

xamarin/Xamarin.Forms#5272
xamarin/Xamarin.Forms#2415 (comment)

[rockfordlhotka]

Closing this, as it will be handled via the Shell implementation in #342