Add option to remove gallery code from the selected codebase

[avivash]

Since all four webnative apps (https://github.com/webnative-examples/webnative-app-template, https://github.com/webnative-examples/webnative-app-template-react, https://github.com/webnative-examples/walletauth, https://github.com/webnative-examples/walletauth-react) are now using a DDD approach for the gallery code, we should add a prompt to this CLI to allow users to decide whether or not they want the gallery code included in the app they generate.

I think we should include the gallery code by default, simply because there isn't much else going on in the apps once the user has signed in, but it is likely many users will not want the gallery code included if they're using the generated templates to build other products.

Notes:
I think it's best to hold off on implementing this until after the 0.2 milestone has been completed, because there may be some UI updates and code shuffling done to the four templates as part of that