Name full-screen-image setup function and call it

This change sets up "Full Screen Image" JS everytime courseware tabs are clicked, as they are js/templates and not available in the DOM on page load.

common/lib/xmodule/xmodule/js/src/html/imageModal.js

-$(function() {
-  
-  // Set up on page load
+function setupFullScreenImage() {
   $("a.modal-content").each(function() {
     var smallImageObject = $(this).children();
     var largeImageSRC = $(this).attr('href');
@@ -106,4 +104,4 @@ $(function() {
       $(".wrapper-modal-image .image-content .image-controls .modal-ui-icon").toggleClass('is-disabled');
     }
   });
-});
+}
\ No newline at end of file

common/lib/xmodule/xmodule/templates/html/image_modal.yaml

@@ -4,4 +4,5 @@ metadata:
 data: |
         <h2>Full Screen Image</h2>
         <p>Use the full screen image tool to allow students to open the image in the full screen, zoom in on the image, and drag the image within the screen.</p>
-        <a href="http://static.class.stanford.edu/stanford-hills-big.jpg" class="modal-content"><img alt="The Stanford Hills" src="http://static.class.stanford.edu/stanford-hills-small.jpg" /></a>
+        <a href="http://static.class.stanford.edu/stanford-hills-big.jpg" class="modal-content"><img alt="The Stanford Hills" src="http://static.class.stanford.edu/stanford-hills-small.jpg" /></a>
\ No newline at end of file
+        <script>$(function () {setupFullScreenImage();});</script>