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