This commit migrates the old issue template to the new format
documented on the following page:
https://help.github.com/en/github/building-a-strong-community/about-issue-and-pull-request-templates
The old "issue template" is now a generic "bug report".
New templates have also been added:
* crash report
* feature request
* question
Providing users with those templates allows reducing redundant
questions to the reporter ("where is the stacktrace", "what is the
usecase" etc), and assigning labels to the created issues
automatically.
A pull request template that refers to the CONTRIBUTING document and
reminds the reporter to create a waiver commit is also implemented
by this commit.
