✨(models/api) add link access reach and role

Link access was either public or private and was only allowing readers.

This commit makes link access more powerful:
- link reach can be private (users need to obtain specific access by
  document's administrators), restricted (any authenticated user) or
  public (anybody including anonymous users)
- link role can be reader or editor.

It is thus now possible to give editor access to an anonymous user or
any authenticated user.

src/backend/demo/management/commands/create_demo.py

@@ -130,7 +130,9 @@ def create_demo(stdout):
             queue.push(
                 models.Document(
                     title=fake.sentence(nb_words=4),
-                    is_public=random_true_with_probability(0.5),
+                    link_reach=models.LinkReachChoices.AUTHENTICATED
+                    if random_true_with_probability(0.5)
+                    else random.choice(models.LinkReachChoices.values),
                 )
             )