Add antispam app

* update_spam_database command to update suspicious domains from an
   external provider
* Add a AntiSpamEmailField that deny emails from suspicious domains
* Update documentation

antispam/models.py

@@ -0,0 +1,19 @@
+from django.db import models
+from django.utils.translation import gettext_lazy as _
+
+
+class ToxicDomain(models.Model):
+    """Domain marked as spam in public databases"""
+
+    domain = models.URLField(_("domain"), max_length=253, primary_key=True)
+    created = models.DateTimeField(auto_now_add=True)
+    is_externally_managed = models.BooleanField(
+        _("is externally managed"),
+        default=False,
+        help_text=_(
+            "True if kept up-to-date using external toxic domain providers, else False"
+        ),
+    )
+
+    def __str__(self) -> str:
+        return self.domain