Mail message needs to call a template defined in hook_theme

Problem/Motivation

Mail message defined in send() method calls a template twig with path of module so we can't personalise it. And the variables called in the twig are not replaced by the render system but are replaced with a str_replace.

Proposed resolution

I made a patch that fixes that problems. With this patch, all websites car write their own template "email_login_otp_mail.html.twig" to write the message they want.