Automatically generate a changelog based on merge request

Proposed by @yorickpeterse in https://gitlab.slack.com/archives/development/p1460983911000134

The problem is that right now the changelog causes merge conflicts, is sometimes forgotten and it doesn't link to the merge requests that contain the conversation about a feature.

1. Add GitLab automatically updates this file on the first line of the changelog and an empty line after that (no project setting needed).
2. Any merge request already generates a merge commit, this commit will also add a new line at the top of the changelog below the first two lines.
3. If the changelog is a markdown files all entries will be text that links to the MR, otherwise it will be only text.
4. The text of every line will be the title of the merge request.
5. You have to add version numbers manually.

/cc @JobV @dzaporozhets

Also see https://github.com/chef/omnibus#changelog suggested by @marin