Add support for repository level webhooks

[drautureau-sonarsource]

Description

Give the ability to subscribe for updates of builds and tasks on a dedicated repository. It is basically the same feature that was implemented at the organisation level https://cirrus-ci.org/api/#webhooks

Context

This allows to have multiple integration specific to a GitHub project

[fkorotkov]

Probably it will make sense to be able to add multiple hooks as well.