Add link to delete a template

This is a link not a button because:
- it’s less prominent—delete is an infrequent action
- it’s a two-step process, and only the second part changes any data (so it has
  a button)

app/templates/views/edit-template.html

@@ -14,7 +14,8 @@ GOV.UK Notify | Edit template
 			{{ textbox(form.template_name) }}
 			{{ textbox(form.template_body, highlight_tags=True) }}
 			{{ page_footer(
-				'Save and continue',
+				'Save',
+				delete_link=url_for('.delete_template', service_id=service_id, template_id=template_id) if template_id or None,
 				back_link=url_for('.manage_templates', service_id=service_id),
 				back_link_text='Back to templates'
 			) }}