Ship disabled GNOME Shell

Originally created by @alant on #6235 (Redmine)

We’ve decided to:

1. Ship with GNOME classic mode.
2. If it doesn’t take too much space, ship GNOME Shell too.

There is a dconf setting to
disable the shell (/org/gnome/session/session-name set to
gnome-fallback.

Last attempt to use it was
config/chroot_local-includes/etc/dconf/db/local.d/00_Tails_defaults,
but it doesn’t seem to be working.

Edited May 15, 2020 by alant

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