Don’t allow adding unknown domains to branding lookup

We should make sure we’re not putting typos in the branding list. We can
validate what gets entered here against our known list of public-sector
domains.

app/main/validators.py

@@ -11,7 +11,7 @@ from wtforms.validators import Email
 
 from app import formatted_list
 from app.main._blacklisted_passwords import blacklisted_passwords
-from app.utils import Spreadsheet, is_gov_user
+from app.utils import AgreementInfo, Spreadsheet, is_gov_user
 
 
 class Blacklist:
@@ -102,3 +102,12 @@ class DoesNotStartWithDoubleZero:
     def __call__(self, form, field):
         if field.data and field.data.startswith("00"):
             raise ValidationError(self.message)
+
+
+class KnownGovernmentDomain:
+
+    message = 'Not a known government domain (you might need to update domains.yml)'
+
+    def __call__(self, form, field):
+        if field.data and AgreementInfo(field.data).owner is None:
+            raise ValidationError(self.message)