New Onboarding: prevent (or warn) about going back to signup after creating a new site

[mattwiebe]

This will be fixed in https://github.com/Automattic/wp-calypso/issues/50435

What

Prevent (or warn) about going back to /new after creating a new site.

Why

The user may be confused, thinking that their site was unsuccessfully created, losing trust in our product

How

A few possibilities:

• an AYS dialog when attempting to navigate back beyond the first post-signup screen
• manipulate the history API to eliminate the possibility of going back to signup
• open the post-signup URL in a new window and close the former one (probably bad)
• detect a recently completed signup and some some kind of notice when starting a new signup

Context

In user testing the Site Editor, an error was encountered in the Site Editor that made it appear that they had somehow accidentally navigated away from the Site Editor. They used the back button to try to get back into the Site Editor but instead they landed back in signup.

You can see it in action at p7DVsv-ayb-p2#comment-34603

[razvanpapadopol]

This issue seems to be similar to https://github.com/Automattic/wp-calypso/issues/49565 with the difference that in this case the user hits back from editor and not from checkout. I believe both are fixed in https://github.com/Automattic/wp-calypso/pull/50327 but worth re-checking.

We are also planning to do some UX improvement showing a dialog to warn the user instead of just redirecting back to editor. See https://github.com/Automattic/wp-calypso/issues/49565#issuecomment-784341234 and the dedicated issue: https://github.com/Automattic/wp-calypso/issues/50435