Corrected subscriber_count to subscribers_count; added integration tests of user public/private status

[jhsu802701]

I added integration tests of the user's ability to switch between public and private status and the user's visibility on a repo page. (At the moment, private subscribers are not shown on the repo pages but public subscribers are.)

Many parts of the code call the subscribers_count method for the repo object, but it was called "subscriber_count" in the repo model and in a few other places. I have replaced all instances of "subscriber_count" with "subscribers_count". The tests of the visibility of a repo's subscribers were failing before I made this change.

[jhsu802701]

I resolved the merge conflict.