Issue #1757566 by alexpott, aspilicious: Convert user account e-mail templates to configuration system.

PR for https://github.com/backdrop/backdrop-issues/issues/170
In all of user_admin_settings(), let's try to keep the variable names consistent. e.g.
```
- '#default_value' => _user_mail_text('register_admin_created_subject', NULL, array(), FALSE),
+ '#default_value' => $mail_config->get('register_admin_created_subject'),
```