Add a data cleanup cron queue

Problem/Motivation

With more statements supported (see 📌 Support the 'answered' verb Active ), and even the previous support, the LRS data tables may grow too large, leading to performance and disk space issues.
Data cleanup is not compliant with Xapi standards as LRS normally keeps all records forever so we should make sure this is done the right way.

Proposed resolution

Quick solution (here): Add a setting on LRS settings form where admins can define for how many days statements are stored and a cleanup cron job (cron queue in batches?). The setting textfield must include a description with a warning that complete data cleanup is not compliant with Xapi standards.

Comprehensive solution (follow-up): Same setting as above but different cron job that moves old records to an archive data table instead of deleting completely. Endpoints will then have a "Include archive" parameter.