Improve the changelog format to reduce noise and improve readability

[nicolaiarocci]

Changelog format can be improved. Progress was made with the 0.8 release, whereas we added active links to relevant tickets, but I feel like it can still be improved. Ideally, I'd like to find a compromise between the guidelines proposed in places like keepachangelog and Flask's changelog, which I like. Some things to consider:

• Rename CHANGES to CHANGES.rst, so it is properly rendered on GitHub and elsewhere
• The same types of changes should be grouped. Right now, all those new, fix, etc. line tags add a lot of noise
• Drop the distinction between development and stable releases. We use semantic versioning anyway.