Add support for "pull_request_status"

[PieterBunnenberg]

We are using the WIP bot in combination with draft prs.

Example:

1. We create draft prs and start developing something
2. If it is ready for review we convert the pr to ready-for-review
3. We might set the do-not-merge label, because we want to block the integration for a given time (using the WIP bot)

Problem:
The combination of draft prs and the WIP bot results in strange combinations that might be confusing to the developers, see:

Solution:
Having a location field pr_status that checks for the pr status and will fail for draft prs would be beneficial, so that the both status are in sync.

[gr2m]

yeah why not, would you like to send a pull request for it?