Allow Secure Files to be disabled at instance-level

What does this MR do and why?

This MR updates the interface and API for Project-level Secure Files to allow customers to disable the feature at the instance level via a configuration setting.

Closes gitlab-org/incubation-engineering/mobile-devops/readme#130 (closed)

How to set up and validate locally

1. Visit the CI/CD Settings page for a project and ensure the Secure Files section is visible (i.e. /flightjs/Flight/-/settings/ci_cd)
2. Edit the gitlab.yml.erb file in support/templates/gitlab/config locally in GDK and switch enabled: false under ci_secure_files
3. gdk reconfigure; gdk restart
4. Reload the CI/CD Settings page for the project and ensure the Secure Files section is no longer visible

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.