release process: formalize the changelog template (#2837)

I did this manually for the last few releases and I think it's going to be
helpful in the future too. Unfortunately this adds a little more work during
the release (sorry @cooperlees).

This change will also improve the merge conflict situation a bit, because
changes to different sections won't merge conflict.

For the last release, the sections were in a kind of random order. In the
template I put highlights and "Style" first because they're most important
to users, and alphabetized the rest.