Fix problem with local search when site_url is not at the root

When the site url is not at the root, the localsearch plugin __init__.py needed to be modified slightly to include the folder when using the post permalink. In addition, I added some corresponding comments to the conf.py.sample.
Let’s not do that.  This is a 1% use case, we’re better off generating files that work for this Nikola site only and fuck everyone else.