Do not replace whole crontab for user in appctl

[Cheaterman]

Hi,

Everything in the title - I had the misfortune of trying to run ./appctl upgrade to upgrade my setup, it erased my whole crontab file for the user.

Thanks in advance!

EDIT: See https://github.com/rafalp/misago_docker/blob/master/appctl#L157

[rafalp]

Hello. Pull request changing this is welcome :)