Unread counts fixes (#8254)

* Fixup `ALTER TABLE` database queries

Make the new columns nullable, because doing otherwise can wedge a
server with a big database, as setting a default value rewrites the
table.

* Switch back to using the notifications count in the push badge

Clients are likely to be confused if we send a push but the badge count
is the unread messages one, and not the notifications one.

* Changelog

synapse/push/push_tools.py

@@ -36,7 +36,7 @@ async def get_badge_count(store, user_id):
             )
             # return one badge count per conversation, as count per
             # message is so noisy as to be almost useless
-            badge += 1 if notifs["unread_count"] else 0
+            badge += 1 if notifs["notify_count"] else 0
     return badge