Introduce structured commit messages.

[KaiserKarel]

Currently it's a little bit freestyle with commit messages. This makes it difficult to

• generate proper release notes
• review a PR
• git-blame/git-bisect

We should move to a model where:

1. Individual commit messages follow semantic commits
2. Each commit has a paragraph with proper English describing the changes in full.

Even if we squash PRs, the commit messages end up in the git history. Doing this ASAP will solve many issues in the future.