Username sanitization silently remove diacritic & such

[Jbb]

As per #13159

Manually adding user (backend User->Add new) allow for any username (for example, Dédé) without any error. But WP silently sanitize the username (Dd in my example), so the admin has no idea something went wrong.

Also, Self registration indicate a “Invalid username” error, without any more explanation. Some textual explanation about what's ok and what not might be helpful.

See r14264 & r14265 for consistency.

[nacin]

I'm going to lower the severity a bit. Self registration is prevented, and the admins can at least look at the user that is created.

Patches welcome.

[sivel]

Is there any reason why we cannot support accented characters? Or perhaps we should at least use remove_accents to convert the characters to their non accented counterparts.

[koopersmith]

If we can't support accented characters, I'd be for using remove_accents and potentially some JS to strip accents as the username is entered (though, without a warning, that might be confusing in its own right).

[Complainer]

A situation where this becomes truly problematic:

1. Create a user and edit his nickname (not username) to include accented characters. If you create a post using this user, the author will be displayed properly on the page, with all foreign characters.
2. Export your posts. In the created XML file, the author of the post will still be saved properly.
3. Import posts on a blank WP installation from created file.
4. New users are created during the import process and their names are broken once again.

[sivel]

Make user validations actually work and tweak the error messages

[sivel]

wpmu_validate_user_signup does not currently use validate_username and should probably be updated.

The importers do not use validate_user either.

Don't commit yet, still researching where else we may need this functionality and where these changes could adversely affect current functionality.

[sivel]

Looks like this should be good. Commit when ready. Let me know if anything looks wrong with the patch.

[ryan]

(In [14428]) Fix user validation feedback. Props sivel. fixes #13162