Block @ in WPORG usernames

[Ipstenu]

We should filter @'s (and email addresses) from WPORG usernames so people can stop signing up as ipstenu@… and so on (like I just did, you can nuke that).

[iandunn]

Why?

(I'm not objecting, just curious, because I don't see any inherent harm in it.)

[Ipstenu]

Because people rarely (if ever) actually WANT a public username to be their email address. It causes spam, and also a high number of people kvetching to us because we can't change the usernames.

tl;dr - Protect people from being silly.

[samuelsidler]

Another solution would be to detect (via JS) an email address in the username field and pop-up a message saying something along the lines of "Your username is public. Are you sure you want it to be your email address? We recommend a username different from your email address."

[Ipstenu]

If we did that, we may as well add it 'Your username is public and cannot be changed....'

Not that I think people actually read, though, Sams. They'll just click and then complain later.

[nacin]

We need to make our usernames more restrictive. Spaces, @ signs, and such cause a lot of inconsistency, grief, and general issues across WP.org.

[Otto42]

The @ signs have given us grief in other ways too. They're blocked now. Existing usernames can still have them, but new accounts cannot.

I took this change and added one more in: ​https://github.com/WordPress/wporg-mu-plugins/commit/abc02533432fb1987ea9b96d5a7fef28c0c51d73.

Thanks for the head start!