Fix #769

Adding branches_to_be_notified to SlackService and deprecating notify_only_default_branch as per
https://docs.gitlab.com/ee/api/services.html#createedit-slack-service

based on docs https://docs.gitlab.com/ee/api/packages.html and inspired by GroupProjectsFilter

Added missing name attribute to Badges class. Also aligned indent of Badges class with rest of api.model package

https://docs.gitlab.com/ee/api/discussions.html#delete-a-merge-request-thread-note

Ensuring test SystemHooks are removed if tests fail, adding commentary/todo to address GitLab issue at future date

In GitLab 11.11, the Project Hook Key for wiki events was wiki_events.
We don't support GitLab 11.X. We only support GitLab 12.9.2+.
In new version of GitLab the correct key is wiki_page_events.