Added switch to count external storage data in quota

This includes all mountpoints except the Shared one in
the used space calculation.

@ser72 this already works but there are probably many hidden bugs, especially related to free space calculation on other user's shared folders.

We could provide this as an experimental/beta feature and fix the bugs as they appear.
The most important thing is that whenever the switch if OFF it should work as before, so no regressions on that.