Follows up #64b35120
This also corrects it so bytes used for internal storage counts all the space
used, previously it was only counting uploads not optimized images.
Additionally we now correctly count storage for optimized images.
This normalizes it so we only carry one place for grabbing disk space size
It also normalizes the command made so it uses Discourse.execute_command
which splits off params in a far cleaner way.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
* Dashboard doesn't timeout anymore when Amazon S3 is used for backups
* Storage stats are now a proper report with the same caching rules
* Changing the backup_location, s3_backup_bucket or creating and deleting backups removes the report from the cache
* It shows the number of backups and the backup location
* It shows the used space for the correct backup location instead of always showing used space on local storage
* It shows the date of the last backup as relative date
