Increase visibility into LFS usage and GC
## Why
It can be hard to get an overview of LFS disk usage, and once implemented it might be hard to tell when LFS GC/pruning has run).
@kforner suggested the following in https://gitlab.com/gitlab-org/gitlab-ce/issues/30639#note_42261773:
>>>
What could be enough to begin with, and at least useful for debugging is a view of a table for a given project with:
- LFS object ID
- LFS object size
- nb of references
and a button to delete unreferenced LFS objects.
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## What
### Rake task
Perhaps a rake task which lists total LFS size for a project followed by a breakdown of the largest objects
### Project usage page/section
Perhaps a section on a page somewhere which lists total LFS usage along with when the GC/pruning background worker last ran.
issue