users: single character usernames rejected

When creating a new user, a username of one character is not accepted. The guidance provided for username is "Required. 150 characters or fewer. English letters, digits and @/./-/_ only."

A new username is checked for a match with regex expression r'^[\w.@][\w.@-]+\Z' which requires a minimum of two characters.

Either the regex should be changed to r'^[\w.@][\w.@-]*\Z' to allow a username of one character (preferred, unless a one-character username presents some other problem), or the guidance should be changed to replace "150 characters or fewer" with "2-150 characters" and add "cannot begin with -".