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.
Don’t do the replacement.  It would be better, safer and faster to have users change it themselves.  Besides, there are some more instances that need this.

Instead, add a comment saying *make sure to set the `<form>`, `<script>` and `<link>` tags and `contentLocation` to the correct path if you are not hosting your website at the server root!* or something like this.