Ability to restrict file size / disk space for LFS

This is something we will want to do for the future of GitLab.com, but is apparently also interesting for CERN (supposedly for others as well).

@marin with the current git hook in EE, can you already do this? I suppose not.

I guess we also want to add the ability to track usage in the UI.

cc @jacobvosmaer @sytses @dzaporozhets