Installing yaru should try to remove the communitheme snap

This would need to happen on new installs of yaru-theme-gtk only, not updates.

The removal should be attempted, but failure to do so for whatever reason (does snapd need network access to remove an installed snap?) should be gracefully ignored.

lxd has some logic in the deb package to install the snap in its preinst script (sort of the opposite of what we're trying to achieve here), this can be interesting: https://github.com/lxc/lxd-pkg-ubuntu/blob/dpm-cosmic/debian/lxd.preinst

The following should do the trick (will require thorough testing):

$ cat debian/yaru-theme-gtk.postinst
#!/bin/sh

set -e

# When first installing yaru-theme-gtk (i.e. when upgrading from bionic),
# remove the communitheme snap if present (https://launchpad.net/bugs/1792604).
if [ "$1" = configure -a -z "$2" ]; then
  snap remove communitheme || true
fi

#DEBHELPER#

Tested this in a PPA, and it works as expected. One minor issue is that when uninstalling the communitheme snap while the theme is in use, icons in the dock change on the fly, and that doesn't look great. That would happen only during upgrade from bionic and to users actually using the communitheme session, so that's probably acceptable, but we can reduce the impact further by having the snap removed in the dist-upgrader code at the very end of the upgrade process (when unused debs are being removed).

And the session files (/usr/share/xsessions/ubuntu-communitheme-snap.desktop and /usr/share/wayland-sessions/ubuntu-communitheme-snap-wayland.desktop) installed by ubuntu-session should move to a separate binary package and be removed when upgrading.

As we discussed, I'm working on the immediate issue: moving to a separate binary package.
This is done on https://bugs.launchpad.net/ubuntu/+source/gnome-session/+bug/1794024