Label folders to show what’s in them

It feels like a solid reckon that knowing what’s in a folder before you
click on it will help you navigate around.

However, what do you show in a folder if you’re filtering by template
type? We think that:
- if you’re not filtering you should see all folders, even empty ones
- if you’re filtering you should only see folders that will get you to
  relevant templates

This matches what happens when you filter templates, we don’t ‘grey out’
the non-email templates, we hide them completely.

The logic then extends to how we describe the contents of a folder, ie
we won’t count its subfolders if they don’t contain templates of the
type we’re looking for. This means that however you see the folder
described (eg ‘3 templates, 1 folder’) will match what you see when you
click into it.

app/templates/views/templates/_template_list.html

@@ -1,4 +1,5 @@
 {% from "components/checkbox.html" import unlabelled_checkbox %}
+{% from "components/message-count-label.html" import folder_contents_count %}
 
 <nav id=template-list>
   {% for template_folder in template_folders %}
@@ -15,7 +16,12 @@
           {{ template_folder.name }}
         </a>
       </h2>
-      <p class="message-type">Folder containing {{ template_count }} template{% if template_count != 1 %}s{% endif %}</p>
+      <p class="message-type">
+        {{ folder_contents_count(
+          current_service.get_template_folders(template_folder.id, template_type)|length,
+          current_service.get_templates(template_type, template_folder.id)|length,
+        ) }}
+      </p>
     </div>
   {% endfor %}
   {% for template in templates %}