Implement Issue and Pull Request Templates for our Github Repo

Recently Github (where MyPaint code is hosted) recently implemented issue and pull request templates. These templates help bug reporters and code contributors either give us the information we need to before we can fix the bug, or a checklist of what is require before we can accept a pull request. I think the community can help us by giving us feedback about how our template should look. One thing we don’t want to do is overwhelm our community from contributing to the MyPaint source code. Let us know your thoughts about this matter down below. It will be a great help in shaping the future of the MyPaint Community.