Hooks for personal notifications

Problem to solve

Notifications in "/profile/notifications" have only an option to email.

Further details

I'd really like the option for a webhook with the intention of sending those notifications to slack (or telegram) instead, as honestly, who uses email any more 😁

Proposal

What does success look like, and how can we measure that?

On a mention, a hook is called with the details of that mention. On thread activity, a hook is called with the details of that thread activity.

etc.