Add hiring justification requirement for rejections

Why is this change being made?

Previously the hiring justification checklist did not mention that past rejections should be noted and explained. This adds instructions for past rejections to the checklist.

This was prompted by discussion in Slack#hire-hs-2: "Our process for the last several years is that the hiring manager needs to confirm with the interviewer, and document in the justification scorecard (or they carefully consider their decision and decide not to go to offer)." More info in a Greenhouse candidate profile: past rejections should be "addressed in the justification scorecard under 'flags raised'".

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