Email templates contain old m1 {{blocks}} to header and footer

### Preconditions
1. Migrate data from M1 to M2 using data-migration-tool.

### Steps to reproduce
1. Open any order in admin panel.
2. Send email by "Send Email".

### Expected result

> You sent the order email.

### Actual result
```
[2020-01-28 15:11:23] main.CRITICAL: Email template '' is not defined. {"exception":"[object] (UnexpectedValueException(code: 0): Email template '' is not defined. at ......mydomain.com/vendor/magento/module-email/Model/Template/Config.php:225)"} []
```

### Additional notes
The problem is that in m2 we use different blocks than in m1.

M1:
```
{{template config_path="storename/email/header"}}
{{template config_path="storename/email/footer"}}
```
M2:
```
{{template config_path="design/email/header_template"}}
{{template config_path="design/email/footer_template"}}
```

This issue on [magento.stackexchange.com](https://magento.stackexchange.com/questions/244926/migrate-magento1-email-templates-to-magento2).

Start with the migration behavior described for data-migration-tool and inspect vendor/magento/module-email/Model/Template/Config.php:225, then reproduce the Send Email flow from an order in the admin panel. Done means migrated templates use the M2 header and footer paths and sending the order email no longer produces an undefined-template error.

Written by the indexing model from the issue text.