Validate name field on ServiceUpdateEmailBranding form

This introduces a validator to validate that the name field is not empty
on the ServiceUpdateEmailBranding form, but only if the form details are
being submitted. If a file is being uploaded, the name is allowed to be
empty.

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

@@ -29,6 +29,8 @@
            {{ radios(form.brand_type) }}
           {{ page_footer(
             'Save',
+            button_name='operation',
+            button_value='email-branding-details',
             back_link=url_for('.email_branding'),
             back_link_text='Back to email branding selection',
           ) }}