fixes PROD-102
When a newsletter has a sender_email stored in the database that Ghost
is not allowed to send from, we no longer return it as sender_email in
the API. Instead we return it as the sender_reply_to. That way the
expected behaviour is shown correctly in the frontend and the API result
also makes more sense.
In addition to that, when a change is made to a newsletters reply_to
address we'll clear any invalid sender_email values in that newsletter.
That makes sure we can clear the sender_reply_to value instead of
keeping the current fallback to sender_email if that one is stored.
On top of that, this change correclty updates the browse endpoint to use
the newsletter service instead of directly using the model.
eb063f7a40