Add new pre-receive secret detection application setting to /admin page

Overview

To complete the work from #428760 (closed), we have to update the UI as outlined in #428637 (closed) to allow an administrator of a GitLab instance to turn the feature on/off.

Implementation Plan

• Add a new expandable section under Admin > Settings > Security and Compliance with the name Secret Detection.
• Under the section, a new checkbox should be available to turn the feature on/off.
• Integrate the checkbox with the application setting pre_receive_secret_detection_enabled.