Don't send stream emails to deleted users

[alastair]

I was looking through some of our logs and saw some checking for sending log emails to a deleted user. This isn't a problem in practise - we don't keep this user's email address, and the checker method that we use to see if we email a user also checks the deleted user flag, but at the very least we should stop trying to do this. Either by undoing the send stream email preference when deleting/anonymising a user, or by checking against the profile is_deleted_user flag when sending stream emails (and also any other time that we do an operation like this)