Add Storage metrics to Control Centre

There is currently no way to reveal the storage usage for an environment.

Add a screen where there is a summary presented

* Database size
* Blob size

Additionally a button to download a spreadsheet of the blob entities stored. Each showing
* file Name
* file ID
* file Size
* environment

The above should include all of the cached Image Config files as well as the originals.

All of the above is important as inRiver are beginning to factor storage into their licencing model as a scaling factor.

Thanks
Mark