enhance webhook granularity to repository level

[xaleeks]

add option to the webbook menu to allow specifying repositories on which to fire webhooks. Some users are overwhelmed by the volume of webhook notifications when enabled for a project

[Vad1mo]

I think we should strive to make the webhook, notification and auditlog functionality similar based on an emited events

In #15363 (comment) I outlined with a few libs how a general purpose webhook system might look like.

[wy65701436]

let’s move it v2.6.