Closes #4627

now it is possible to config the cache via ./constants.local.php

<?php
	define('CLEANCACHE_HOURS', 240);

default: 720 hours

Changes proposed in this pull request:

• introduce the constant CLEANCACHE_HOURS

How to test the feature manually:

1. check the content of the folder ./data/cache
2. create ./constants.local.php and set the hours
3. go to the frontend and reload the page (it would help to comment out the condition if (mt_rand(0, 30) === 1) in lib/lib_rss.php)
4. old cache files are deleted in ./data/cache

Pull request checklist:

• clear commit messages
• code manually tested