When must_be_following_dm is on, only notify if recipient dm'ed user (#6283)

* When must_be_following_dm is on, only notify if recipient dm'ed user
Currently, when must_be_following_dm is on, if a user sends a direct
message replying to any status from the recipient, the recipient gets a
notification. This should not be the case, as if the recipient posted
something publicly this can be used to spam their notifications.

* Refactor replied_to_status_is_direct_message?
Following suggestion in PR