Add preferences-system-disks icon
Somewhat of an alternative to !291.
These 22px and 32px icons support the "Disks & Cameras" group in System
Settings that currently uses the drive-harddisk icon, which is always
symbolic at the size it's displayed at in System Settings. We want it to
be colorful instead.
Downsides to this approach: 3rd-party icon themes without this icon would fall back to preferences-system until they get one.
