disks: Add removable media automounting

`udiskie` is kept running in the background and automatically
mounting any inserted media, which will then also show its
tray icon so that it can be quickly unmounted as well.

Additional ideas would be to allow mounting / unmounting through
keyboard sequences instead of gui (via rofi for example).

disks/.config/systemd/user/default.target.wants/udiskie.service

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+../udiskie.service

disks/.config/systemd/user/udiskie.service

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+[Unit]
+Description=Automated mounting of removable devices (udisks)
+
+[Service]
+# workaround to allow relative executable invocation (i.e. current users' home dir)
+ExecStart=/usr/bin/udiskie
+Restart=always
+
+[Install]
+WantedBy=default.target

disks/.config/udiskie/config.yml

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+program_options:
+  tray: auto
+  automount: true
+  notify: true
+device_config:
+# ignore any snap loopback devices
+# see https://github.com/coldfix/udiskie/issues/180
+- id_type: squashfs
+  ignore: true

disks/README.md

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+# Removable disk configuration
+
+This simple unit depends on `udiskie2` being installed and enables auto-mounting of any inserted removable media.
+It will additionally *ignore* any snaps managed by `snapd`, since they would otherwise be detected as loopback devices.
+
+Automatically installs a `systemd` service which is also enabled to start up on system start (as a user service).
+
+The end result is `udiskie` running in the background and automatically mounting any inserted media, 
+which will then also show its tray icon so that it can be quickly unmounted as well.