Prepare for upcoming changelog workflow changes

This adds a configuration file for generating changelogs using the GitLab API (done using Release Tools), as well as change Danger to remind developers of these upcoming changes (similar to what we're doing in gitlab-org/gitlab).

Omnibus, Pages and Helm are already using this new approach. Our current plan is to also roll this out to remaining projects (GitLab Rails and Gitaly) per May 22nd.

See gitlab-com/gl-infra/delivery#1564 (closed) for more information.