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.
Nikola will handle the following for you:

```html
<form action="/search.html">
<script src="/assets/js/tipuesearch_set.js"></script>
<script src="/assets/js/tipuesearch.js"></script>
<link rel="stylesheet" type="text/css" href="/assets/css/tipuesearch.css">
```

However, we can’t do a thing about:

```js
        'contentLocation': '/assets/js/tipuesearch_content.json',
```

I think the best new solution for this would be to use string concatenation — something like:

```py
BODY_END = """
[… snip …]
         'mode': 'json',
        'contentLocation': '""" + SITE_URL + """assets/js/tipuesearch_content.json',
[… snip …]
"""
```

This will break on sites with SITE_URL that doesn’t end with a trailing slash, something we warn about on every run on bad sites.  (I just made it say *change it* explicitly)