Don’t highlight ((double brackets)) in broadcast templates

You can’t use them to personalise the message, so it’s confusing for
the UI to suggest they’re somehow special.

app/templates/views/edit-broadcast-template.html

@@ -24,7 +24,7 @@
         }) }}
       </div>
       <div class="govuk-grid-column-two-thirds">
-        {{ textbox(form.template_content, highlight_placeholders=True, width='1-1', rows=5) }}
+        {{ textbox(form.template_content, highlight_placeholders=False, width='1-1', rows=5) }}
         {{ sticky_page_footer('Save') }}
       </div>
     </div>

tests/app/main/views/test_templates.py

@@ -399,6 +399,21 @@ def test_should_show_page_for_one_template(
     mock_get_service_template.assert_called_with(SERVICE_ONE_ID, template_id, None)
 
 
+def test_broadcast_template_doesnt_highlight_placeholders(
+    client_request,
+    service_one,
+    mock_get_broadcast_template,
+    fake_uuid,
+):
+    service_one['permissions'] += ['broadcast']
+    page = client_request.get(
+        '.edit_service_template',
+        service_id=SERVICE_ONE_ID,
+        template_id=fake_uuid,
+    )
+    assert page.select_one('textarea')['data-highlight-placeholders'] == 'false'
+
+
 def test_caseworker_redirected_to_one_off(
     client_request,
     mock_get_service_templates,