Name full-screen-image setup function and call it

Fix bug in fullscreen image where JS doesn't load when you switch unit tabs in LMS.
Maybe we want to do this, to be safe?
```javascript
$(function () {
    setupFullScreenImage();
});
```

I'm not familiar with how the yaml templates are loaded/rendered, but could we hit a race condition here where this function was executed before before the (hoisted) func declaration is evaluated?