Using pull requests for all code changes

[scop]

Now that we have more people actively involved, let's start using pull requests for all code changes.

I think I'd also like to switch to using merge commits instead of rebasing, but extra/make-changelog.py likely wouldn't do a too good job with that. We should probably switch also to some other tooling for generating the changelogs, I bet a bunch of ready made applicable ones exist as/if we're moving towards using conventional commits commit messages (should document that in CONTRIBUTING.md BTW, and likely add a commitlint pre-commit check).

Thoughts?