Make separate endpoints for GOV.UK email branding options

The endpoint to change the email branding to "GOV.UK" branding and
"GOV.UK and organisation" branding was the same but with a query string
used to determine which of the two options had been selected. This makes
them two separate endpoints, which makes the code a bit simpler and
hopefully means there is less chance of things not working as expected.

tests/app/test_navigation.py

@@ -112,6 +112,7 @@ EXCLUDED_ENDPOINTS = tuple(map(Navigation.get_endpoint_with_blueprint, {
     'edit_user_permissions',
     'email_branding',
     'email_branding_govuk',
+    'email_branding_govuk_and_org',
     'email_branding_nhs',
     'email_branding_organisation',
     'email_branding_request',