Provide interface for turning feature flags on and off in the review app

[owenatgov]

What

Provide a way to easily switch between feature flag states in the review app ie: not just than manually changing a sass variable to true and reloading.

@colinrotherham has already written a brief investigation into what this could look like #4086 (comment)

Why

So that we can easily see and test changes between feature flag states.

Done when

• Investigate different ways to present feature flags and technical feasibility of each
• Choose a way to present feature flags
• Apply chosen solution