[FIX] website_form: send email with subject
[FIX] website_form: send email with subject

- Use the form builder to send an email
- Customize the snippet to include the subject and the body
- Send a mail

=> The subject is not included in the email

This is because the subject is filtered out and considered as a
blacklisted field.

It happens because `mail.mail` inherits from `mail.message`. The field
property `website_form_blacklisted` is set to:
- `True` for `mail.mail`
- `False` for `mail.message`

Since we go through all inherited models, the property on `mail.mail` is
overriden by the property on `mail.message`.

It is actually not necessary to go through all inherited models, since
in the case of `inherits`, we copy the fields of the inherited model to
the target model. The check is simplified by only checking the fields of
the current model.

Based on work of @nla-odoo

opw-748926

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo