♻️(backend) refacto email invitation

Remove email invitation from Invitation model
to be able to use it in other context.
We add it in utils.py instead, and it will be called
from the viewset.
We add the document_id to link to the document from
the mail.

src/mail/mjml/invitation.mjml

@@ -30,7 +30,7 @@
               <li>{% trans "Invite members of your community to your document in just a few clicks."%}</li>
             </ul>
           </mj-text>
-          <mj-button href="//{{site.domain}}" background-color="#000091" color="white" padding-bottom="30px">
+          <mj-button href="//{{site.domain}}/docs/{{document_id}}/" background-color="#000091" color="white" padding-bottom="30px">
             {% trans "Visit Docs"%}
           </mj-button>
           <mj-text>{% trans "We are confident that Docs will help you increase efficiency and productivity while strengthening the bond among members." %}</mj-text>