allow TAs to select the cohort of new posts.

TNL-153

@andy-armstrong @dan-f reqd
@gwprice FYI
to answer the question you actually asked, checkVisibility is something added in this change to bundle up a few assertions about the control whose state interests us.  it doesn't verify visibility of the whole view, just that selector, and when we expect it to be visible, we also make sure it's enabled.