Convert system/exec usages to Symfony's Process class

[cpeel]

Replace all applicable uses of exec, passthru, shell_exec and system with Symfony's Process class. The package/class was introduced into the codebase in https://github.com/DistributedProofreaders/dproofreaders/pull/1273

Add a CI script that checks these functions aren't used in the code with an approve-list for files we've allowed as an exceptions. Since this is security related, we might change check_security.sh into a PHP file similar to check_includes.php and put it in there but other approaches are fine.

[cpeel]

Note: iterative work on this issue is appreciated, it doesn't need to be solved all at once.

[mrducky4]

I will start working on this.