✨(models/api) allow inviting external users to a document by their email

We want to be able to share a document with a person even if this person
does not have an account in impress yet.

This code is ported from https://github.com/numerique-gouv/people.

src/mail/mjml/partial/header.mjml

@@ -14,10 +14,8 @@
                 font-family="Roboto, -apple-system, BlinkMacSystemFont, 'Segoe UI', Oxygen, Ubuntu, Cantarell, 'Helvetica Neue', sans-serif"
                 font-size="16px"
                 line-height="1.5em"
-                color="#031963"
+                color="#3A3A3A"
         />
-        <mj-class name="text--small" font-size="0.875rem" />
-        <mj-class name="bg--blue-100" background-color="#EDF5FA" />
     </mj-attributes>
     <mj-style>
         /* Reset */
@@ -33,7 +31,7 @@
     <mj-style>
         /* Global styles */
         h1 {
-            color: #055FD2;
+            color: #161616;
             font-size: 2rem;
             line-height: 1em;
             font-weight: 700;