Add ARIA attributes from Design System component

This commit copies the same ARIA attributes that are added to the
character count component[1] in the GOV.UK Design System.

This means that screen reader users will hear the count message when
they stop typing.

1. https://design-system.service.gov.uk/components/character-count/

tests/app/main/views/test_templates.py

@@ -484,6 +484,12 @@ def test_should_show_page_for_one_template(
         template_type='sms',
     )
 
+    assert (
+        page.select_one('[data-module=update-status]')['aria-live']
+    ) == (
+        'polite'
+    )
+
     mock_get_service_template.assert_called_with(SERVICE_ONE_ID, template_id, None)
 
 
@@ -518,6 +524,12 @@ def test_broadcast_template_doesnt_highlight_placeholders_but_does_count_charact
         template_type='broadcast',
     )
 
+    assert (
+        page.select_one('[data-module=update-status]')['aria-live']
+    ) == (
+        'polite'
+    )
+
 
 def test_caseworker_redirected_to_one_off(
     client_request,