Fix API log page in Firefox

The details of each notification were not being hidden on page load in
Firefox.

Firefox does not natively support the `<details>` element, so we
polyfill it.

Because of the way the polyfill is written[1]

1. There can’t be any `<div>` elements inside the `<summary>` (this
   commit changes them to be `<span>`s instead, and adds CSS to make
   sure they wrap as before)
2. The contents to be shown/hidden must be wrapped in a `<div>` (which
   this commit adds)

***

1. https://github.com/alphagov/govuk_elements/blob/48fde82c721917feed3b7cf9b655e41fcddb3f49/public/javascripts/govuk/details.polyfill.js#L90

app/assets/stylesheets/views/api.scss

@@ -19,6 +19,7 @@
     }
 
     &-meta {
+      display: block;
       color: $secondary-text-colour;
     }