User-configurable cron expression for regular cache flushing

[LukasCesal]

The current cron expression from version 1.7.x is fixed at 30 2 * * * (daily cache flush), which may not suit more busy instances.
The goal is to be able to customize this cron expression.

Summary (*)

Examples (*)

Proposed solution

I suggest configuring cron using system settings (config_path instead of cron_expr).