Add way to preview closed surveys as if they were open

[SachaG]

Add URL parameter (or keyboard shortcut, UI control, etc.) to make it possible to preview closed surveys as if they were open for screenshots, talks, etc.

[eric-burel]

We could add a search parameter to the "/outline" version:

• getting searchParams from the page props loses staticness, because you can't generate static pages based on them, it's a bit annoying
• useSearchParams would work but requires client-side rendering. That's acceptable and should probably be the preferred way.

Then we need an additional boolean compared to "readOnly", being "readOnly" and "disabled" would be treated slightly differently. As a default readOnly => disabled but we could force components being enabled.

That touches many files for a limite use case so let's treat it as low priority, alternative is to temporarily make the survey hidden in dev.