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/forms.py

@@ -35,6 +35,7 @@ from app.main.validators import (
     Blacklist,
     CsvFileValidator,
     DoesNotStartWithDoubleZero,
+    KnownGovernmentDomain,
     LettersNumbersAndFullStopsOnly,
     NoCommasInPlaceHolders,
     OnlyGSMCharacters,
@@ -722,7 +723,7 @@ class ServicePreviewBranding(StripWhitespaceForm):
 class ServiceUpdateEmailBranding(StripWhitespaceForm):
     name = StringField('Name of brand')
     text = StringField('Text')
-    domain = StringField('Domain')
+    domain = StringField('Domain', validators=[KnownGovernmentDomain()])
     colour = StringField(
         'Colour',
         validators=[