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/components/message-count-label.html

@@ -55,3 +55,26 @@
     {%- endif -%}
   {%- endif %}
 {%- endmacro %}
+
+
+{% macro folder_contents_count(number_of_folders, number_of_templates) %}
+
+  {% if number_of_folders == number_of_templates == 0 %}
+    Empty
+  {% endif %}
+
+  {% if number_of_templates == 1 %}
+      {{ number_of_templates }} template
+  {%- elif number_of_templates > 1 -%}
+      {{ number_of_templates }} templates
+  {%- endif -%}
+
+  {%- if number_of_folders and number_of_templates %}, {% endif -%}
+
+  {%- if number_of_folders == 1 -%}
+      {{ number_of_folders }} folder
+  {%- elif number_of_folders > 1 -%}
+      {{ number_of_folders }} folders
+  {% endif %}
+
+{%- endmacro %}