Add Feature Flag

[bycdiaz]

• [ ] Add feature flag
• [ ] Add placeholder content (content we're hiding)
• [ ] Add test confirming users with flag can observe hidden content
• [ ] Add test confirming users without flag cannot observe hidden content.

Acceptance Criteria

1. Enable flag for your current user
2. Navigate to x
3. Observe hidden content

[JustWaveThings]

From meeting on 5/11:

• Set up user feature flag check as one first backend tasks and frontend tasks
  • Feature flag conditional: https://github.com/TheOdinProject/theodinproject/blob/a04874d606f4bae3dddf03b684c14855c66972e8/app/views/static_pages/home.html.erb#L24