PR and review templates

[rvolosatovs]

As this project is maturing we should focus more on code quality and not just making new things work, but also ensuring that the "old" things continue working as well. This should, of course, be automated as much as possible, which is an ongoing process, see e.g. #38

As a low hanging fruit, we should add meaningful PR templates with checklist of things that have to be done an tested, as well as a checklist for reviewers to fill in on review or at least a well-defined process to follow.