Make the empty folder state more useful

Currently if there’s nothing in a folder you just get an empty page.
This looks a bit broken, or like the page hasn’t finished loading.

This commit adds a message to the page to show that it’s intentionally
blank.

The message is contextual based on type of template, because there might
be templates in the current folder, even if you can’t see them at the
moment (because you’re filtering).

app/models/service.py

@@ -133,7 +133,10 @@ class Service():
 
     @property
     def has_templates(self):
-        return len(self.all_templates) > 0
+        return bool(self.all_templates)
+
+    def has_folders(self):
+        return bool(self.all_template_folders)
 
     @property
     def has_multiple_template_types(self):