Make deletion confirmation banner messages consistent across our app

Also introduce a way to provide context to a banner / flash message
that will be displayed in plain font style.

tests/app/main/views/test_api_integration.py

@@ -310,7 +310,7 @@ def test_should_show_confirm_revoke_api_key(
     assert normalize_spaces(page.select('.banner-dangerous')[0].text) == (
         'Are you sure you want to revoke this API key? '
         '‘some key name’ will no longer let you connect to GOV.UK Notify. '
-        'Confirm'
+        'Yes, revoke'
     )
     assert mock_get_api_keys.call_args_list == [
         call(