Add `pages_multiple_versions_enabled` to `ProjectSetting`
As part of the introduction of the GitLab Pages Multiple Versions Support, we need to introduce a kill switch of the feature, where the user can enable/disable the multiple version of GitLab Pages. This way the project owner has the power to disable the feature and avoid contributors to add multiple versions of the site.
This feature is GitLab Premium.
Edited by Kassio Borges