Allow creating a GitHub draft release without generating release notes

[sgarner]

Description

Some users won't need np to auto-generate release notes from commits, since GitHub now has a feature to do this when creating a release or they might have another method of their own. So it would be helpful to have an option to disable this functionality.

This would also provide a solution to the issue in #501 where the release notes generated by np may exceed GitHub's URL length limit and prevent the draft release page from loading.

Possible implementation

I propose that a --no-release-notes flag could be added to tell np not to generate release notes when opening the GitHub release draft.