Allow generating preloader from cron/external link

Problem/Motivation

Right now the only way to generate a preloader is to access the admin dashboard, which isn't possible in some cases, we need the way to generate a preloader script from the script, to make it part of the deployment procedure or periodically update it by cron.

Proposed resolution

Add secured link like for Cron run that re-generating preloader script, it can be called from deployment script or from corn.

I've already done many tests with generating preloader from Drush, but it must be generated from FPM/mod_php process and not from CLI because it creates the wrong preloader which includes classes added in CLI mode only and not adding any really needed, like Request/Response classes.

Remaining tasks

Add tests. Maybe add link to Reports/Status dashboard.

User interface changes

On settings form add permanent link for external run.