Some issues with staged users and their (in)ability to visit topics

[zogstrip (Régis HANOL)]

@Sander78

Simplest solution: have staged users get custom copy in their mail notifications. No visit this topic, just reply by mail.

It’s now done

https://github.com/discourse/discourse/commit/21b51090bfaef940f2b050c8a4d05246b1824bef