DeletionWatcher: answers are never re-watched upon reboot

[makyen]

Rewatching an answer upon reboot currently depends on GlobalVars.post_site_id_to_question[post_site_id] to have the question ID for the answer. That is only populated in spamhandling.py and is not stored persistently (i.e., it's empty upon reboot). It would be better to store the question ID in the datawatcher pickle, so it doesn't need to be discovered upon reboot (and the question ID isn't available from the SE API unless there's at least one comment (it's available in the link provided for the comment).

[codygray]

Is this now closed, with the merging of #11839?

[makyen]

@codygray No, that PR explicitly doesn't resolve this issue, #11539, or #11540.