MM-58255 Ensure remote users do not get valid email addresses (#27421)

* remote users don't get valid email addresses; remote users cannot have access tokens

* block notification emails for remote users

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/user.go

@@ -346,7 +346,7 @@ func (u *User) IsValid() *AppError {
 		}
 	}
 
-	if len(u.Email) > UserEmailMaxLength || u.Email == "" || !IsValidEmail(u.Email) {
+	if len(u.Email) > UserEmailMaxLength || u.Email == "" || (!IsValidEmail(u.Email) && !u.IsRemote()) {
 		return InvalidUserError("email", u.Id, u.Email)
 	}