Use templates for form data emails

This PR allows using templates for emailing form data.
There must be a file named justl like a form available in the `custom/emails` folder.

Use case: pre-processing inquiries

Form name: `foobar`
File: `custom/emails/foobar.php`
Template body:
```html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<body>
Field foobar: <?= $foobar ?>
</body>
</html>
```

Current template [build method](https://github.com/aheinze/cockpit/blob/0.12.0/modules/core/Forms/bootstrap.php#L54) may be hard to read for end users when used complex data structures.

Note: referencing docs: [Forms](https://github.com/aheinze/cockpit-docs/blob/1d45cb5824810be620d3a7e9fb4ecd600546bac9/modules/forms.md)
I'll change it to `custom:forms/emails` then, it sounds quite explicit.