Second submit newsletter

https://www.pivotaltracker.com/story/show/82978346
I found there is no change in my first submission. So I resubmit it. 
I have do some modification following your suggestions. I check the initialized variables, and the updates values after calling the method. Because I couldn't call the function explicitly, I just try to set the mode to instant, and do_send to be true, which might send newsletters for me automatically. However, I can only check the last_user_id is exactly the last person's id. I set the CHUNK_SIZE 100, but I cannot prove any difference, if there are more people than 100. I really appreciate your review and help.
there is inconsistent use of `stub_const` and `const_set` here, in some places, `const_set` is used and in others, `stub_const` is used (like in [line 89](https://github.com/AgileVentures/WebsiteOne/pull/607/files#diff-d7cc21b5ba78fbb9caed79d73082ec66R89)). It is better to keep it consistent and use only one approach, it is confusing because both methods achieve the same thing.