Set default branding for NHS services

The NHS is a special case because it’s not one organisation, but it does
have one consistent brand. So anyone working for an NHS organisation
should have their default branding set when they create a service, even
if we know nothing about their specific organisation.

app/utils.py

@@ -122,3 +122,9 @@ def escape_special_characters(string):
             r'\{}'.format(special_character)
         )
     return string
+
+
+def email_address_is_nhs(email_address):
+    return email_address.lower().endswith((
+        '@nhs.uk', '@nhs.net', '.nhs.uk', '.nhs.net',
+    ))