Use the static variables set in config.php instead

All paths specified now are using the static variables set in ``config.php`` for ease of future maintenance, and site configuration (if someone chooses to run Cindy from a non-standard directory). In addition, added a variable for the plugins folder.

Modifications have been made to these functions, among other minor changes:
* the ``template_file()`` function in ``app/asset-types/page.inc.php``
* the constructor in ``app/cache.inc.php``
* the ``file_cache()`` and ``site_last_modified()`` functions in ``app/helpers.inc.php``
* the ``create_collections()`` and ``create_textfile_vars()`` functions in ``app/page-data.inc.php``
* the ``collate_partials()`` function in ``app/parsers/template-parser.inc.php``
* ``app/stacey.inc.php``
* ``config.php``