Create a letter branding request flow to match email branding request

Test if service settings links to branding request page for letters

Parametrize all branding tests so they also work for letter branding

app/models/service.py

@@ -417,6 +417,12 @@ class Service(JSONModel):
             return 'GOV.UK'
         return self.email_branding['name']
 
+    @cached_property
+    def letter_branding_name(self):
+        if self.letter_branding is None:
+            return 'no'
+        return self.letter_branding['name']
+
     @property
     def needs_to_change_email_branding(self):
         return self.email_branding_id is None and self.organisation_type != Organisation.TYPE_CENTRAL