Document how to re-enable Bluetooth in the GNOME session

This is step 2 from https://mailman.boum.org/pipermail/tails-ux/2017-September/003454.html i.e.: document how to re-enable Bluetooth in the GNOME session and with Additional Software Packages (see commits 4fa1c46, 613b14c, 3d53d2c and 9320ca7 for inspiration).

Devices that are needed to log in via the Greeter (keyboard, mouse) are out of scope, which decreases the priority/usefulness of this ticket quite a bit, but still, at least one user has been asking for it since we did #14655 (closed).

When we work on this, if the new set of commands needed to re-enable Bluetooth is substantially harder to remember and type by hand than the old one (rfkill + apt install), IMO we should provide a wrapper script.

Related issues

Related to #10801
Related to #14655 (closed)

Original created by @intrigeri on 14957 (Redmine)

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