jinja2: Use function interface for render_markdown_path.

Apparently, the filters implementation was doing some sort of strange
caching, where you would need to restart the server in order to
refresh for changes to the markdown content.

We fix this by switching to just calling the render_markdown_path
function from Jinja2.

Fixes #5974.

zproject/jinja2/__init__.py

@@ -20,6 +20,7 @@ def environment(**options):
     env.globals.update({
         'static': staticfiles_storage.url,
         'url': reverse,
+        'render_markdown_path': render_markdown_path,
         'minified_js': minified_js,
     })
 
@@ -28,6 +29,5 @@ def environment(**options):
     env.filters['slugify'] = slugify
     env.filters['pluralize'] = pluralize
     env.filters['display_list'] = display_list
-    env.filters['render_markdown_path'] = render_markdown_path
 
     return env