Simplify our feature flags with the unleash service
Summary
Replaces register pages, onboarding and new homepage feature flags with a single one, called 'ux-2020'
Closes #1318 (closed)
Steps to test
With 'ux-2020' enabled, the following should show:
- new homepage
- new onboarding
- new login and register pages
With the FF disabled, the old ones should appear
Estimated Regression Scope
None