Updated title and template for image modal html component

LMS-2215

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

 ---
 metadata:
-    display_name: Image Modal
+    display_name: Full Screen Image
 data: |
-        <h2>Title of Unit (click image to zoom)</h2>
+        <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>
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