[UDisks] Ignore non-user mounts

This gets rid of all those internal mounts, such as /snap mounts.
The approach is similar to GVFS which ignores everything outside /media, /home/user, and /media/run,
cf. https://github.com/GNOME/gvfs/blob/master/monitor/udisks2/what-is-shown.txt
Explicitly creating a mount in /etc/fstab outside of those directories is still possible and will always show up.

BUG: 379516
CCBUG: 319998

CHANGELOG: Storage devices mounted outside of /media, /run/media, and $HOME are now ignored, as well as Loop Devices whose
backing file is outside of $HOME

Differential Revision: https://phabricator.kde.org/D9895