Transition GitLab pages to object storage instead of NFS

First part of the refactoring: &1316 (closed)

• Add support for object storage as a backend in gitlab-pages
• Add ability to create a .zip file during the pages deploy process
• Add ability to deploy to object storage during the pages deploy process
• Create a migration strategy to move sites from file storage to object storage

Note: Don't forget to use project name independent URLs for object storage(see hashed storage). discussion

Possible ways to implement

Serve directly from zip artifacts

See gitlab-pages!136 (closed) for and idea

PoC serving from object storage using pre-signed URLs gitlab-pages!299 (closed)

Download zip artifacts by pages and use current disc serving

Build CDN between object storage and pages

Open questions

will that allow as to get rid of jailing?