Make the webserver's rentention time for log files configurable

This feature ditches the current static 30 days approach of logfile purging and introduces a configurable retention time for each website.
The retention time configuration can be found in the options tab of the website settings.

Default retention time is the current 30 days value.

However, by this approach it's possible to have retention times like 0 (forever), or very short retention times like 7 days which are often demanded by public/government data security officials which we have in germany.