Add more storage statistics

What does this MR do?

Display more information about used storage:

• display total repository size for groups (admin/groups#index and admin/groups#show currently)
• include build artifacts
• include LFS objects
• expose counters in API
• update counters when creating/destroying LFS objects
• update counters when creating/destroying CI builds
• handle shared LFS objects (ignored for now, see this comment)
• use .badge style after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7873 is merged

This development is sponsored by @siemens (ping @bufferoverflow)

Are there points in the code the reviewer needs to double check?

Answered in comment below:

• Not sure if ProjectCacheWorker is the most appropriate place to update the namespace cache, or if I should add a NamespaceCacheWorker
• Not sure yet how I should display the other object types, it probably makes sense to store them in dedicated columns, but display a total in most places in the interface

Why was this MR needed?

• Currently the storage information is only displayed per project, making it inconvenient to find out how much storage a whole group is using
• The storage information is only for the bare Git repository, other things like builds and uploads are not included

Screenshots

Admin group overview, displays total storage:

Admin project overview, displays total storage:

Admin group detail, displays total and individual counters:

Admin project detail, displays total and individual counters:

User project view, displays repository size + LFS objects:

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/18352
• https://gitlab.com/gitlab-org/gitlab-ce/issues/17611

Info: Updating storage counters after update

The new storage counters will be updated when necessary, to force an update for specific columns run the following in gitlab-rails console:

ProjectStatistics.includes(:project).find_each(batch_size: 100) do |stats|
  stats.refresh! only: [:build_artifacts_size, :lfs_objects_size]
end