Object store for artifacts

How it works?

(Kamil) This is revisited implementation of Object Storage. It stores information, where the file is stored and by default, continues using local storage for artifacts. However, it implements a rake task to move artifacts to external storage and host it from there. This is manual rake task but allows us to do background migration of artifacts (in the future also have a cron job to do that).

Thus this allows us to work on removing limitation mentioned in Workhorse merge request, but start moving data and doing rollout testing of this feature.

This also shows a way for us to later to allow artifacts to be configured on a per-project or per-group basis.

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce!9860 (now a EEP feature)

Closes gitlab-org/gitlab-ce#29203