Synchronize CHANGELOG.md from gitlab-ce

We accidentally merged an unresolved conflict into the CHANGELOG.md file in the last CE to EE merge. While looking at this, I also noticed that our CHANGELOG.md file has gotten out of sync with the CE version of the file (which should be identical!). Likely this was through bad automatic conflict resolutions in past ce-to-ee merges. I'll open a MR to simple sync the CE and EE CHANGELOG.md file.