hack around future unburied date

We want to avoid unburied != self.today, because the unburied time
is synchronized as part of the collection at the moment, and we don't
want a client with an older unburied time that was modified more
recently to cause cards to be unburied twice - so we only unbury
if today is more than 7 days earlier than the last unbury time.

Unsure what caused these users to end up in that state in the first
place:
https://anki.tenderapp.com/discussions/ankidesktop/41335-cards-remain-buried-forever-unless-manually-unburied

pylib/anki/schedv2.py

@@ -1260,7 +1260,7 @@ where id = ?
 
         # unbury if the day has rolled over
         unburied = self.col.conf.get("lastUnburied", 0)
-        if unburied < self.today:
+        if unburied < self.today or self.today+7 < unburied:
             self.unburyCards()
             self.col.conf["lastUnburied"] = self.today