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.

See merge request !576

instead we are now using dynamic purging of the logs.

re-added fix to the letsencrypt cron job



See merge request !556

udate lng-files: "Send outgoing copy to" is used in sender_bcc_maps



See merge request !554

Stable 3.1



See merge request !536

Stable 3.1



See merge request !535

changed template for nginx + LE



See merge request !526

Stable 3.1



See merge request !525

update create_list in resync



See merge request !522

check if the database exists in db_update



See merge request !520