For anyone keeping track at home, we decided to refactor this in the following way: 

1. There is no longer a default mode wherein you can use the querystring to activate showcase mode. Whether on Shiny Server or locally, you must manually opt in to showcase mode to make it available.

2. The `AllowShowcaseModeOverride` and `DefaultShowcaseMode` settings in `DESCRIPTION` have been collapsed to a single setting, `DisplayMode`. If this setting has the value `Showcase`, then the app defaults to showcase mode by default, and the app's showcase mode can be toggled on the querystring.