Link template folders to their contents

Clicking on template folder navigates to a page that displays that
folder's contents.

This reuses the existing choose template view by adding a filter
based on optional `template_folder_id` argument.

Service model methods are rewritten to match `all_templates` and
`get_template`. New `get_template_folder_path` method returns a
list of folders (from root to the current one) that the selected
folder is nested inside.

app/templates/views/templates/choose.html

@@ -13,7 +13,7 @@
 
 {% block maincolumn_content %}
 
-  {% if not templates %}
+  {% if (not templates) and (not template_folder_path) %}
 
     <h1 class="heading-large">{{ page_title }}</h1>
     {% if current_user.has_permissions('manage_templates') %}
@@ -62,9 +62,9 @@
       {% for template_folder in template_folders %}
         <div class="column-whole">
           <h2 class="message-name">
-            {# <a href="{{ url_for('.view_template_folder', service_id=current_service.id, template_folder_id=template_folder.id) }}"> #}
+            <a href="{{ url_for('.choose_template', service_id=current_service.id, template_type=template_type, template_folder_id=template_folder.id) }}">
               {{ template_folder.name }}
-            {# </a> #}
+            </a>
           </h2>
           <p class="message-type">Folder containing {{ template_count }} template{% if template_count != 1 %}s{% endif %}</p>
         </div>