Object store for artifacts

Closes gitlab-ce#29203

See merge request !1762

[ci skip]

This adds counters for build artifacts and LFS objects, and moves
the preexisting repository_size and commit_count from the projects
table into a new project_statistics table.

The counters are displayed in the administration area for projects
and groups, and also available through the API for admins (on */all)
and normal users (on */owned)

The statistics are updated through ProjectCacheWorker, which can now
do more granular updates with the new :statistics argument.

[ci skip]

- Split user and admin documentation
- Use new location paths
- Add new Continuous Integration guide for the Admin area
- Link to new guide from the Admin area