Make 'Add Changelog/License/Contribution Guide' buttons optional

This was done by adding new (changelog|license|contributing)_hint_enabled members to the projects model for each of the respective 'Add ...' buttons that appear on the Projects page, and updating the view/controller to match. The project schema, API, and project default features were updated to match.

The motivation is that when GitLab is used internally, some projects will not be following the conventional model and may have a specific file structure that shouldn't include these files.

Project Settings page

Project homepage (with only "Contribution Guide" selected)