Create an admin area view to show LFS disk usage

Zendesk: https://gitlab.zendesk.com/agent/tickets/25926

Customer is going to begin rolling out LFS to projects. They're worried that if they just open the flood gates they will run out of disk space quickly. They would like to be able to track the total LFS disk usage across all projects from an admin area dashboard.

Apparently we have some of this info in the database, so it may be possible to query the DB for the data.

cc/ @DouweM @JobV