Create version of Gallery app that uses Fabric instead of Paper

[chrisglein]

The long-term goal here is that the Gallery app transitions completely to Fabric. As that the Gallery app contains a bunch of community modules, many of which may not have Windows versions on Fabric for a period of time, there needs to be an incremental plan here. Something like this:

1. Create a "forked" version of Gallery on Fabric that only contains the core components
2. Launch this to the store as a private flight, but do not replace the main published version (which needs to continue to work and may need to be updated)
3. Run a draft accessibility grading pass on the app, tracking requirements blocking it becoming the main version
4. As community modules start to get Windows support, re-enable them in the Gallery app
5. When sufficient module support exists, replace the main store app with the Fabric version

One of the challenging aspects of this is that it'd need react-navigation.

[chrisglein]

Duplicate of #384