Merge proposal "needs review" emails can be sent before the diff is generated

Setting an MP to Needs Review creates an UpdatePreviewDiffJob and a MergeProposalNeedsReviewEmailJob. The Needs Review email contains the preview diff, if present, so its job needs to run after the UpdatePreviewDiffJob. Pre-celery this works fine, as the two job types are run by a single serialised runner. But in the celery world the jobs can be run simultaneously, and the email is sent before the diff is generated, so the diff is not attached.

I'm not quite sure how to fix this. Possibly the MPNREJ should be created SUSPENDED. The UPDJ can then unsuspend it when it completes. But if the UPDJ fails we still need to unsuspend the MPNREJ, possibly via a celerybeat task that just unsuspends any MPNREJ over a certain age.