Nav Unification - implement menu fallback experience

[getdave]

This is a Sub Issue of #45435.

If the menu data is to come from a API request then we need to consider what happens when that request fails.

Calypso's data layer has a built in retry mechanic, but ultimately we should have a fallback experience for any time that the API fails or the cache is empty.

[getdave]

@marekhrabe suggested that we save a static snapshot of the API endpoint's output and render from that immediately and then hydrate from fresh dynamic data as and when available.