lowercase_username() is not mb_string safe

[Denis-de-Bernardy]

ms-functions.php has a lowercase_username() function, which is an alias for strtolower.

Are we sure that the username is UTF7? If not, we should make sure that it is.

I'm pretty shure that Denis meant 7Bit-Ascii and not ​UTF-7.

lowercase_username() and strtolower_usernames() were yanked in [12948] because they both duplicated strtolower().

Should we be accounting for mb strings here? sanitize_user() removes all non-ascii characters.