Simplify our feature flags with the unleash service

With the compiled work of onboarding, product pages and nav, we need to reduce the number of feature flags to turn on the new ui in one toggle. We should also take the time to clean up our existing feature flags and make them all configurable or remove the invalid ones

• setup a new feature flag called 'ux-2020'
• remove the onboarding, product page and nav feature flags
• audit the feature flags in engine
• make sure all current feature flags are setup in the feature flag export mapping
• enable toggling of the feature flag on and off with the unleash client