Do not notify draft PR has merge conflicts

[alexanderwu-db]

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I have notifications turned on for PR status (the Slack integration: Integrations -> Scheduled reminders). It notifies me when my PR has merge conflicts. This feature isn't that useful in the first place as I usually already know, or will check it and find out eventually. But it even notifies me for draft PRs. If it's a draft, clearly it's not ready for merge anyway, so I should not care about merge conflicts IMO. I would highly prefer if we were not notified about merge conflicts on draft PRs.

[marc-hb]

This is likely a request for Slack more than for Github.

BTW I wish conflict notification worked "out of the box" (as in Gitlab?) and did not require Slack in the first place.

• Notify when merge conflict arises isaacs/github#224