Make the new Draft PR behavior configurable

[ThiefMaster]

If you look at #682 and the underlying issue #680 it's clear that this is highly organization-specific:

Hello, within our organization, we commonly utilize draft pull requests for experimental ideas and prototyping, and convert these drafts pull requests once they are ready for review.

For example, in my case we open Draft PRs simply because e.g. tests have not been fully updated yet or there are still some discussion points.

And there I'd much rather have it show the full message when the PR is opened, and nothing at all when it's changed from/to draft (like it was before #682).

In particular, it's spammy right now when someone opens the PR, then converts it to draft (because they forgot), and then back to a regular one shortly after (that results in a total of two "full" messages and one small).

I think this change would be much more bearable if the plugin kept track of whether a PR was in "ready" state before so the bull message is only sent once. But even then I'd rather have a setting to disable the whole functionality, because depending on the project, Draft PR just means that there's still some stuff left for discussion, but that people still want to be properly informed about the PR...