Registration: Reject usernames that contain "www."

[cweiske]

Wordpress registration e-mails are used to send bitcoin spam to people - by using usernames with spaces like

www.spammer.example.com - 1.2342 BTC

This patch filters out usernames that begin with "www." or contain " www." - such names are auto-linked by email clients and made clickable.

Documented occurences of that spamming problem:

• https://cweiske.de/tagebuch/wordpress-registration-spam.htm
• https://www.reddit.com/r/Wordpress/comments/1gyyhx7/spammed_with_100_fake_wordpress_login_emails_help/

Fixes #63085.

Trac ticket: https://core.trac.wordpress.org/ticket/63085

Use of AI Tools

AI assistance: No

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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Props cweiske.

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[github-actions]

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[cweiske]

I've added a unit test.

[cweiske]

The tests failure is a flake; it has nothing to do with my code.