fix(celery): use crontab to schedule gc tasks

this should prevent some initial load when beat is started
because it schedules all tasks right away
Merge pull request #427 from petrjasek/feat-cron

fix(celery): use crontab to schedule gc tasks
SD-1222 - fix notification, send proper item
Merge pull request #444 from amagdas/sd-1222

SD-1222 - fix notification, send proper item
Merge remote-tracking branch 'origin/master' into feat-workflow