Get path to HTML Purifier cache from app
Get path to HTML Purifier cache from app

The app has a reusable method that tells us where
HTML Purifier should put the cache files.

Use it instead of trying to get the cache path from the config.

Fixes #472

mdwheele/opencfp,mdwheele/opencfp,PHPBenelux/opencfp,PHPBenelux/opencfp,GrUSP/opencfp,opencfp/opencfp,GrUSP/opencfp,localheinz/opencfp,mdwheele/opencfp,PHPBenelux/opencfp,localheinz/opencfp,PHPBenelux/opencfp,GrUSP/opencfp,localheinz/opencfp,opencfp/opencfp,localheinz/opencfp,opencfp/opencfp,GrUSP/opencfp,mdwheele/opencfp