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.
You're right; this does look like it would work _now_. That said, I do
worry that if the order of things changed (i.e. we included the function
declaration _after_ its invocation), this would break. We should try not
to build in implicit dependencies on the ordering of `<script>` tags,
when we can avoid it, and we can do that here.

Plus, how cool is _this_?

![cool guy with belt and suspenders](http://ediscovery.typepad.com/photos/stock_photos/BeltandSuspenders.png)