Background notifications

[lgrahl]

It may be interesting to receive notifications for messages even in case the tab is not currently opened. This can be done using a ServiceWorker.

Controversial feature since it moves Threema Web into a ServiceWorker that may or may not be active even if the tab has been closed.
Slightly related to #407.

[dbrgn]

Honestly I'm quite creeped out by apps like Telegram Web running in the background without me ever having approved that. I don't think this is a good idea without a dedicated interface (like a browser extension) where those connections can be handled and stopped if desired.

It would also mean that messages are stored in memory even if the Threema Web tab is closed. This would be a privacy risk, the user loses a bit of control over that data. Right now, the user can simply close the tab and all data is gone.

[lgrahl]

Hence the controversial note.