Store what messages are read

Signed-off-by: Emmy D'Anello <emmy.danello@animath.fr>

chat/models.py

@@ -200,6 +200,14 @@ class Message(models.Model):
         verbose_name=_("content"),
     )
 
+    users_read = models.ManyToManyField(
+        'auth.User',
+        verbose_name=_("users read"),
+        related_name='+',
+        blank=True,
+        help_text=_("Users who have read the message."),
+    )
+
     def get_author_name(self):
         registration = self.author.registration