Generate no links by setting ticket number to 0

This is a new approach to solve the problem with changelog entries having no issue. It is an updated version of @hynek's pull request #1. I know you were not happy about the hack. So why do I create an updated pull request?

* AFAIK it's not possible to have roles in docutils that have no or empty arguments. So it won't be possible to have a role with an optional ticket number, like you proposed in TODO.
* Optional options would be possible with directives. But as you wrote in TODO there is the downside of getting very huge entries compared to what we have now. And I agree that the current list of items is as simple as it can and should be.
* Line items without an issue role have problems: You can't set the ``major`` keyword and therefore they go always into bugfix releases - even if you wanted something different!

If this pull request looks like a good solution for you, you can reject my other pull request #3 because this solution totally fits my needs.