Instead of using non-portable features of GNU ln command, such as -T
and --relative, use Python os.path.relpath function to handle it. This
fixes installation failure on FreeBSD.
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