Add Course size, Backup size and Private files size reports

At present there no ways to find where Moodle server disk space is actually going. There are some reports floating around.
These could be tidied up and put into core.

Some relevant discussions: https://moodle.org/mod/forum/discuss.php?d=264090

The wonderful folk at Catalyst may have a report as indicated here: https://moodle.org/mod/forum/discuss.php?d=228167#p990905

This is a third relevant discussion: https://moodle.org/mod/forum/discuss.php?d=212532

I've set the topic to include three things: Course size, Backup size and Private files. But it really needs someone with a bit more intimate idea of things to help formulate the exact best item to do.