Mail preview and send

[didoda]

This provides some tools to have a "Mail preview" section on objects, with a send mail feature.

Usage example follows.
Adjust your object configuration view, like:

{
  "core": [
    "title",
    "body"
  ],
  "mail_preview": {
    "_element": "Form/mail_preview"
  }
}

Set your mail template in body, i.e.:

<p>Gentile {{ user.name }} {{ user.surname }},</p>
<p>benvenuto sulla piattaforma</p>

When you save, in "Mail preview" you see a form to try to send your mail.

Note: your API should provide a /placeholders/send, as the form tries a POST against it, to send the email.