[11.9] Revert "Remove HipChat integration from GitLab"

See merge request gitlab-org/gitlab-ce!27257

Removes confusing/unnecessary checkboxes when trying to configure an
integration. If there is only one supported event we don't need to
allow these to be individually disabled since the integration can be
disabled instead.

E.g. Project Integrations for GitHub, Bugzilla, Asana, Pipeline emails and Gemnasium

Allows integrations to override which triggers are configurable