accounts-service: Add accounts service vendor extension for app filter

Use accounts service’s vendor extension support for storing the app
filter per user, in a way that persists and which access to is
controlled by polkit (so writes can be restricted to administrators).

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://phabricator.endlessm.com/T23858

tools/meson-make-symlink.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -eu
+
+# this is needed mostly because $DESTDIR is provided as a variable,
+# and we need to create the target directory...
+
+mkdir -vp "$(dirname "${DESTDIR:-}$2")"
+if [ "$(dirname $1)" = . ]; then
+        ln -vfs -T "$1" "${DESTDIR:-}$2"
+else
+        ln -vfs -T --relative "${DESTDIR:-}$1" "${DESTDIR:-}$2"
+fi