Previously, we hide when authenticated.
We still need the `isAuthenticated` flag, because `hasPremiumPlus` stays at the unfetched (`undefined`) state for Incognito.
## Issue
- Due to the timer used, the script could be loaded after the path-checking useEffect ran.
- Stale closure is also an issue.
## Approach
Create refs for the props, and listen to `onload` as well to resolve the visibility state.
- Only add for unauthenticated users for now. We can change that to look at premium later without having to touch `app/view.jsx`.
- Only show the ad after knowing the ad is filled; this prevents the invisible container from blocking stuff while waiting, or even worse, when not filled.
