The cost/benefit analysis was tricky to convey that some features almost
come for free once we have others.

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The subkey used for the old signatures expires in a week, on
2018-01-11.

Will-fix: #15143

Thanks cacahuatl!

Partially revert the previous commit.

Let's hope this avoids creating too many duplicates.

Thanks Loïc Dachary for the report that lead to this improvement.