Add webhook alert attributes to REST API for projects/groups

Proposal

The backend here could be fairly simple if we just added some attributes to the REST API for project/group hooks (see: https://gitlab.com/gitlab-org/gitlab/blob/master/lib/api/project_hooks.rb and https://gitlab.com/gitlab-org/gitlab/blob/master/ee/lib/api/group_hooks.rb - centrally controlled by https://gitlab.com/gitlab-org/gitlab/blob/master/lib/api/entities/hook.rb) - then we can just poll the relevant endpoint in frontend and show the alert.