Simplify and standardize site notifications

[pjc09h]

The current (broken) notification system relies on an ancient version of the noty library that depends on jQuery and is also deprecated. Notifications should be simple and leverage existing functionality: email, browser push, and private message.

The configuration should likewise be simple: do you want any combination of the above, or none at all? It's completely pointless to manage, e.g., JavaScript popups for torrents and private messages for comment quotes.

[pjc09h]

The notification options should be simple: private message, push via https://pushjs.org, and email. Some granularity is needed for a good user experience, to avoid spamming people due to coarse controls.