✨(backend) add server-to-server API endpoint to create documents

We want trusted external applications to be able to create documents
via the API on behalf of any user. The user may or may not pre-exist
in our database and should be notified of the document creation by
email.

src/mail/mjml/invitation.mjml

@@ -22,17 +22,11 @@
       <mj-section mj-class="bg--white-100" padding="30px 20px 60px 20px">
         <mj-column>
           <mj-text align="center">
-            <h1>
-              {% blocktrans %}
-                {{sender_name}} shared a document with you !
-              {% endblocktrans %}
-            </h1>
+            <h1>{{title|capfirst}}</h1>
           </mj-text>
           <!-- Main Message -->
           <mj-text>
-            {% blocktrans %}
-              {{sender_name_email}} invited you with the role "{{role}}" on the following document : 
-            {% endblocktrans %}
+            {{message|capfirst}}
             <a href="{{link}}">{{document.title}}</a>
           </mj-text>
           <mj-button