[Media Display] Add a feature flag for new media display

[mplorentz]

Since the new media display gallery will be build over several PRs and releases we want to add a feature flag to turn it on and off. This way the code for the new gallery can be merged but users won't see it in production until it is all done. The feature should be off in production builds, and on by default in debug builds. It's up to the implementer if they would like to add a toggle to our Settings page in debug mode to toggle the feature on and off at runtime.