ContentListView Modal Improvements

Fix bug when creating a ContentListView with `opts.modal: false`. Content with a single attachment, which clicked, would like this:
![bug in action](http://d.pr/i/DzGP+)

Now that stray bottom thumbnail doesn't show.

Also fixes a bug when constructing a ContentListView with `opts.modal: true`. Now this will properly enable Modal support just like not passing `opts.modal` (enabled is/was the default).