Show warnings in comment (for now: regression PR not against master)

[wilzbach]

A first attempt to show helpful warnings (or errors) to a PR submitter (#8).

• In the first commit this adds warnings.d where in the future more warning generator can lay. This commit doesn't modify the output.

• The second commit extends the bugzilla query by requesting more attributes (e.g. severity)

• The third commit combines the first two and emits a warnings when a critical/major/regression/blocking PR isn't based onto stable

Again this is meant as first iteration and we probably have to fine tune this (and can hopefully add more warning generators).
One refactoring step that we should do in any case is to find a common "Cache" format for all information already requested by the diverse APIs - passing them around as function parameters doesn't scale that well.

[dlang-bot]

Thanks for your pull request, @wilzbach! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.

Some things that can help to speed things up:

• smaller, focused PRs are easier to review than big ones

• try not to mix up refactoring or style changes with bug fixes or feature enhancements

• provide helpful commit messages explaining the rationale behind each change

Bear in mind that large or tricky changes may require multiple rounds of review and revision.

Please see CONTRIBUTING.md for more information.

[wilzbach]

@MartinNowak I will merge this as well now, because we agreed that we want to show messages and this PR has been hanging in the queue for four months now & today I have my dlang-bot squash morning which I use to monitor the effect of our deployed changes.