This reverts commit 62d1f0acbd and moves
the current changelog to CHANGELOG-OLD.md.
The current format is pretty messy and wasn't really much better than a
git shortlog. Changelogs are meant for end-users to know what changed
between versions. Most probably won't care much about refactors but may
be interested in UI changes, feature additions/removals, and maybe
client API changes (for 3rd party clients).