See PHI1059. If you close a task, we apply an "alice closed a subtask: X" transaction to its parents.
This transaction is purely informative, but currently requires CAN_EDIT permission after T13186. However, we'd prefer to post this transaction anyway, even if: the parent is locked; or the parent is not editable by the acting user.
Replace the implicit CAN_EDIT requirement with no requirement.
(This transaction is only applied internally (by closing a subtask) and can't be applied via the API or any other channel, so this doesn't let attackers spam a bunch of bogus subtask closures all over the place or anything.)