Add support for confidential issues

[mutezero]

As requested by ajgeiss0702#0702 on Discord.

POST data is the same for both confidential and non-confidential issues. X-Gitlab-Event is different, however - Confidential Issue Hook instead of Issue Hook. There's a object_attributes.confidential field (values: true/false), however, when testing "Confidential issues events", GitLab might send an event for a non-confidential issue.