storage: make external disk mounts accessible by other users

Currently, only root user can read/traverse mounted disk path. This issue suggest that all users can do it. It is needed for Samba sharing #1681 (closed) (EDIT: samba solved it by doing chmod o+rx /media/root). Also related to #1410 (closed).

Mount filesystems to a shared directory so that other users can traverse path.
Udev rules file from Arch Linux wiki [1]:

# UDISKS_FILESYSTEM_SHARED
# ==1: mount filesystem to a shared directory (/media/VolumeName)
# ==0: mount filesystem to a private directory (/run/media/$USER/VolumeName)
# See udisks(8)
ENV{ID_FS_USAGE}=="filesystem|other|crypto", ENV{UDISKS_FILESYSTEM_SHARED}="1"

Make mounted VFAT/FAT32 partition writable to everyone because Windows filesystems don't support setting file permissions (NTFS partitions are mounted world-writable by default).
Udiskie service configuration:

# This file is managed and overwritten by Plinth.
mount_options:
- id_type: vfat             # match file system type
  options: umask=0000       # list of mount options, overwrite default umask 0002

I have tested this configuration in Debian 10/Buster.

Links:
[1] https://wiki.archlinux.org/index.php/Udisks#Mount_to_/media_(udisks2)