Allow sending a custom acknowledgement message to the person submitting this form

[JeffersonBledsoe]

This PR changes the 'send' field to be a multi-select, letting you select 'recipient' or 'acknowledgement'. When acknowledgement is selected, a dropdown for which field to send the acknowledgement appears (and if there is only one email field will be pre-filled with that field). Then a rich text field is visible below letting the editor create a custom message to send. This custom message is then sent to the user filling out the form

Required collective/collective.volto.formsupport#24

Sample

### Email sent to recipient

From: noreply@plone.org
To: jeff.bledsoe@pretagov.com.au
Subject: Test
Reply-To: noreply@plone.org
Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Date: Tue, 13 Dec 2022 13:56:51 +0000


  <p>A new form has been submitted from <strong>Volto form block (custom send=
 to)</strong>:</p>
  <ul>
   =20
      <li>
          <strong>Text:</strong> Test
      </li>
   =20
      <li>
          <strong>My email field:</strong> jeff.bledsoe@pretagov.co.uk
      </li>
   =20
  </ul>

Email sent sent as acknowledgement

From: noreply@plone.org
To: jeff.bledsoe@pretagov.co.uk
Subject: Test
Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Date: Tue, 13 Dec 2022 13:56:52 +0000

<p>This is an email to <a href=3D"http://jeffersonbledsoe.com/" rel=3D"noopene=
r noreferrer">the submitter</a></p><p>Thank <strong><u>you</u></strong> for s=
ubmitting this form!</p><p><em>All the best,</em></p><p><em>Jeff</em></p>