service workers and push - [v96]

in v96 it will be possible to isolate service workers, so we can stop disabling them and switch to that. this also means that we can bring back push notifications in the browser (functionality gained and less fingerprintable).

note to self:

• remove dom.serviceWorkers.enabled
• add privacy.partition.serviceWorkers
• remove dom.push.enabled and dom.push.serverURL
• update faq entry for push notifications. it just need to say that for push to work you need service workers and they must be allowed with an exception (just like cookies).