Allow `!restart` command to work for Bubbles itself

[TimJentzsch]

We can factor out this logic into a separate function and reuse it:

Bubbles/bubbles/commands/update_and_restart.py

Lines 115 to 123 in 6888e73

	# spawn a new child that is separate from the parent process so that it doesn't
	# die immediately as we respawn the parent
	# https://stackoverflow.com/a/16928558
	subprocess.Popen(
	shlex.split(f"sudo systemctl restart {USERNAME}"),
	stdout=open("/dev/null", "w"),
	stderr=open("logfile.log", "a"),
	preexec_fn=os.setpgrp,
	)