Implement PWA push notifications

see e. g. https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Tutorials/js13kGames/Re-engageable_Notifications_Push - is that realistic (and desirable)?

changed milestone to /AlekSIS%"[NLnet #1 (closed)] Implement timetable notifications"

added part::frontend source::customer::fss type::feature workflow::discussing labels

assigned to @yuha