Create a new application setting for pre-receive secret detection

Overview

As we had determined GitLab Dedicated requires an instance-level configuration to allow customers to turn the pre-receive secret detection feature on/off for their instance, and in line with the MVC scope, we will have to add a new application setting that will then be available to update via the UI, later via #428761 (closed).

Implementation Plan

• Decide the name of the application setting, e.g. pre_receive_secret_detection_enabled.
• Create a new boolean column under application_settings table. See this merge request for an example.
• Update the push check from #427039 (closed) to honor this application setting (i.e. if enabled, check runs).