Make page title semi-consistent

We tend to use ‘Add’ rather than ‘Create’ in other places.

app/templates/views/email-branding/manage-branding.html

@@ -9,7 +9,7 @@
 
 {% block platform_admin_content %}
 
-  <h1 class="heading-large">{{ '{} email branding'.format('Update' if email_branding else 'Create')}}</h1>
+  <h1 class="heading-large">{{ '{} email branding'.format('Update' if email_branding else 'Add')}}</h1>
   <div class="grid-row">
     <div class="column-three-quarters">
       {% if logo %}