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/forms.py

@@ -0,0 +1,18 @@
+import re
+
+from django import forms
+from django.core.validators import EmailValidator
+from django.utils.translation import gettext_lazy as _
+
+from antispam.models import ToxicDomain
+
+
+class AntiSpamEmailField(forms.EmailField):
+    """An email field that email addresses with a known toxic domain."""
+
+    def run_validators(self, value: str):
+        super().run_validators(value)
+        # Domain part should exist since email validation is guaranteed to run first
+        domain = re.search(EmailValidator.domain_regex, value)
+        if ToxicDomain.objects.filter(domain=domain[0]).exists():
+            raise forms.ValidationError(_("Email domain is not allowed."))