Bug 1650980: Allow filebrowser to show the size of sub folders

Allow recursive counting so that:

When in 'subfolder A' we get size of 'sub-sub folder A' and when we
are in parent of 'subfolder A' we get size of both.

behatnotneeded

Change-Id: Iea7600a7c46205fc2fe315c2c4d231cc31f83e96
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>