One-click Walkthroughs setup (NEST-529)

[Piedone]

To allow people to use our tutorial from the Walkthroughs module super easily, let's have a one-click setup:

1. You go to the dotnest.com homepage (or perhaps some other page we can link) and click a button to set up a new site with the Walkthroughs recipe. A warning is displayed that the site will be deleted in 24 hours.
2. A site is set up with a random subdomain (like walkthroughs123, but with a longer number).
3. You get redirected to the site's homepage and can being the walkthrough.

After 24 hours, such sites are automatically deleted by a background task.

Jira issue