Make components for nicely formatted lists

We have a couple of places now where we want nice lists made from `list`s, eg

- ‘name’, ‘date’ and ‘phone number’
- ((firstname)) ((lastname)) or ((date))

This commit adds a more generic component for doing this, which can handle:

- 1, 2, and n items
- comma (or other character) separated lists
- a conjunction between the last and one-before-last item
- characters to be inserted before and after each item, eg an opening and
  closing HTML tag

It also pulls the `list_of_placeholders` component from the breaking change
page, and makes it use the `formatted_list` component under the hood.

app/templates/views/styleguide.html

@@ -10,6 +10,7 @@
 {% from "components/textbox.html" import textbox %}
 {% from "components/file-upload.html" import file_upload %}
 {% from "components/api-key.html" import api_key %}
+{% from "components/list.html" import formatted_list %}
 
 {% block page_title %}
   Styleguide – GOV.UK Notify
@@ -230,4 +231,23 @@
 
   {{ api_key('d30512af92e1386d63b90e5973b49a10') }}
 
+
+  <h2 class="heading-large">Formatted list</h2>
+
+  <p>
+    {{ formatted_list('A', prefix="one item called") }}
+  </p>
+
+  <p>
+    {{ formatted_list('AB', prefix_plural="two items called") }}
+  </p>
+
+  <p>
+    {{ formatted_list('ABC') }}
+  </p>
+
+  <p>
+    {{ formatted_list('ABCD', before_each='<strike>', after_each='</strike>', conjunction='or') }}
+  </p>
+
 {% endblock %}