Revert "Usernames must start with a letter (#5581)"

This reverts commit 2e911b77c3.

model/user.go

@@ -10,7 +10,6 @@ import (
 	"net/http"
 	"regexp"
 	"strings"
-	"unicode"
 	"unicode/utf8"
 
 	"golang.org/x/crypto/bcrypt"
@@ -152,7 +151,7 @@ func (u *User) PreSave() {
 	}
 
 	if u.Username == "" {
-		u.Username = "n" + NewId()
+		u.Username = NewId()
 	}
 
 	if u.AuthData != nil && *u.AuthData == "" {
@@ -582,10 +581,6 @@ func IsValidUsername(s string) bool {
 		return false
 	}
 
-	if !unicode.IsLetter(rune(s[0])) {
-		return false
-	}
-
 	for _, restrictedUsername := range restrictedUsernames {
 		if s == restrictedUsername {
 			return false