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/components/list.html

@@ -0,0 +1,37 @@
+{% macro formatted_list(
+  items,
+  conjunction='and',
+  before_each='‘',
+  after_each='’',
+  separator=', ',
+  prefix='',
+  prefix_plural=''
+) %}
+  {% if items|length == 1 %}
+    {{ prefix }} {{ before_each|safe }}{{ (items|list)[0] }}{{ after_each|safe }}
+  {% elif items %}
+    {{ prefix_plural }}
+    {% for item in (items|list)[0:-1] -%}
+      {{ before_each|safe -}}
+      {{ item -}}
+      {{ after_each|safe -}}
+      {% if not loop.last -%}
+        {{ separator -}}
+      {% endif -%}
+    {% endfor %}
+    {{ conjunction }}
+    {{ before_each|safe -}}
+    {{ (items|list)[-1] -}}
+    {{ after_each|safe }}
+  {%- endif %}
+{%- endmacro %}
+
+
+{% macro list_of_placeholders(placeholders) %}
+  {{ formatted_list(
+    placeholders,
+    before_each="<span class='placeholder'>((",
+    after_each='))</span>',
+    separator=' '
+  ) }}
+{%- endmacro %}